We Keep Coding

Comparison arrow object between two bar containers

So I have the following two arrays:
base = np.arange(2)
y_axis = [32.59, 28.096]
And the following code
base = np.arange(2)
fig,ax = plt.subplots()
fig.set_figheight(10)
fig.set_figwidth(15)
bars = ax.bar(base, y_axis, width = 0.3)
bars[0].set_color('g')
ax.bar_label(bars,[f'{i}%' for i in y_axis])
ax.set_xticks(base, labels = ['Simplificado','Não simplificados'])
ax.arrow(base[0],y5,dx = base[1], dy = x5-y5)
That results in the following image
What I want to do is a comparison, arrow something kinda like this. Any ideas on a way to build up such arrow?
Sorry for bad image.

You could use matplotlib.path.
That can be used to draw polygons or also just a polyline following a specific path as used for this case.
This plot isn't optimized to look pretty (see notes at the end for potential improvement), but to show the concept:
Code:
import numpy as np
import matplotlib.pyplot as plt
import matplotlib.path as mpath
base = np.arange(2)
y_axis = [32.59, 28.096]
fig, ax = plt.subplots()
fig.set_figheight(10)
fig.set_figwidth(15)
path_y_gap = 5
delta_value = y_axis[1] - y_axis[0]
Path = mpath.Path
path_data = [
(Path.MOVETO, (base[0],y_axis[0])),
(Path.MOVETO, (base[0],y_axis[0]+path_y_gap)),
(Path.MOVETO, (base[1],y_axis[0]+path_y_gap)),
#(Path.MOVETO, (base[1],y_axis[1])), # alternative to the arrow
]
codes, verts = zip(*path_data)
path = mpath.Path(verts, codes)
x, y = zip(*path.vertices)
line, = ax.plot(x, y, 'k-')
ax.text( 0.5 , y_axis[0] + path_y_gap + 0.5, round(delta_value,2))
ax.arrow(base[1], y_axis[0]+path_y_gap, 0, -(-delta_value + path_y_gap),
head_width = 0.02 , head_length = 0.8, length_includes_head = True)
bars = ax.bar(base, y_axis, width = 0.3)
bars[0].set_color('g')
ax.bar_label(bars,[f'{i}%' for i in y_axis])
ax.set_xticks(base, labels = ['Simplificado','Não simplificados'])
Notes:
path doesn't offer arrow shaped ends, as a workaround the last section is done by a normal matplotlib arrow
Check the alternative in the path_data to the arrow for the last section
I haven't dealt with overlay of the bar % text and the path / arrow, but you could e.g. easily put a y-offset variable to start/end above that text
Check Bézier example in the matplotlib path tutorial if you prefer a 'rounded' line
You may for sure adapt the float digits another way than the used round()
The first MOVETO sets the starting point, an explicit endpoint isn't required.

Is there a way to improve the line quality when exporting streamplots from matplotlib?

I am drawing streamplots using matplotlib, and exporting them to a vector format. However, I find the streamlines are exported as a series of separate lines - not joined objects. This has the effect of reducing the quality of the image, and making for an unwieldy file for further manipulation. An example; the following images are of a pdf generated by exportfig and viewed in Acrobat Reader:
This is the entire plot
and this is a zoom of the center.
Interestingly, the length of these short line segments is affected by 'density' - increasing the density decreases the length of the lines. I get the same behavior whether exporting to svg, pdf or eps.
Is there a way to get a streamplot to export streamlines as a single object, preferably as a curved line?
MWE
import matplotlib.pyplot as plt
import numpy as np
square_size = 101
x = np.linspace(-1,1,square_size)
y = np.linspace(-1,1,square_size)
u, v = np.meshgrid(-x,y)
fig, axis = plt.subplots(1, figsize = (4,3))
axis.streamplot(x,y,u,v)
fig.savefig('YourDirHere\\test.pdf')

In the end, it seemed like the best solution was to extract the lines from the streamplot object, and plot them using axis.plot. The lines are stored as individual segments with no clue as to which line they belong, so it is necessary to stitch them together into continuous lines.
Code follows:
import matplotlib.pyplot as plt
import numpy as np
def extract_streamlines(sl):
# empty list for extracted lines, flag
new_lines = []
for line in sl:
#ignore zero length lines
if np.array_equiv(line[0],line[1]):
continue
ap_flag = 1
for new_line in new_lines:
#append the line segment to either start or end of exiting lines, if either the star or end of the segment is close.
if np.allclose(line[0],new_line[-1]):
new_line.append(list(line[1]))
ap_flag = 0
break
elif np.allclose(line[1],new_line[-1]):
new_line.append(list(line[0]))
ap_flag = 0
break
elif np.allclose(line[0],new_line[0]):
new_line.insert(0,list(line[1]))
ap_flag = 0
break
elif np.allclose(line[1],new_line[0]):
new_line.insert(0,list(line[0]))
ap_flag = 0
break
# otherwise start a new line
if ap_flag:
new_lines.append(line.tolist())
return [np.array(line) for line in new_lines]
square_size = 101
x = np.linspace(-1,1,square_size)
y = np.linspace(-1,1,square_size)
u, v = np.meshgrid(-x,y)
fig_stream, axis_stream = plt.subplots(1, figsize = (4,3))
stream = axis_stream.streamplot(x,y,u,v)
np_new_lines = extract_streamlines(stream.lines.get_segments())
fig, axis = plt.subplots(1, figsize = (4,4))
for line in np_new_lines:
axis.plot(line[:,0], line[:,1])
fig.savefig('YourDirHere\\test.pdf')

A quick solution to this issue is to change the default cap styles of those tiny segments drawn by the streamplot function. In order to do this, follow the below steps.
Extract all the segments from the stream plot.
Bundle these segments through LineCollection function.
Set the collection's cap style to round.
Set the collection's zorder value smaller than the stream plot's default 2. If it is higher than the default value, the arrows of the stream plot will be overdrawn by the lines of the new collection.
Add the collection to the figure.
The solution of the example code is presented below.
import matplotlib.pyplot as plt
import numpy as np
from matplotlib.collections import LineCollection # Import LineCollection function.
square_size = 101
x = np.linspace(-1,1,square_size)
y = np.linspace(-1,1,square_size)
u, v = np.meshgrid(-x,y)
fig, axis = plt.subplots(1, figsize = (4,3))
strm = axis.streamplot(x,y,u,v)
# Extract all the segments from streamplot.
strm_seg = strm.lines.get_segments()
# Bundle segments with round capstyle. The `zorder` value should be less than 2 to not
# overlap streamplot's arrows.
lc = LineCollection(strm_seg, zorder=1.9, capstyle='round')
# Add the bundled segment to the subplot.
axis.add_collection(lc)
fig.savefig('streamline.pdf')
Additionally, if you want to have streamlines their line widths changing throughout the graph, you have to extract them and append this information to LineCollection.
strm_lw = strm.lines.get_linewidths()
lc = LineCollection(strm_seg, zorder=1.9, capstyle='round', linewidths=strm_lw)
Sadly, the implementation of a color map is not as straight as the above solution. Therefore, using a color map with above approach will not be very pleasing. You can still automate the coloring process, as shown below.
strm_col = strm.lines.get_color()
lc = LineCollection(strm_seg, zorder=1.9, capstyle='round', color=strm_col)
Lastly, I opened a pull request to change the default capstyle option in the matplotlib repository, it can be seen here. You can apply this commit using below code too. If you prefer to do so, you do not need any tricks explained above.
diff --git a/lib/matplotlib/streamplot.py b/lib/matplotlib/streamplot.py
index 95ce56a512..0229ae107c 100644
--- a/lib/matplotlib/streamplot.py
+++ b/lib/matplotlib/streamplot.py
## -222,7 +222,7 ## def streamplot(axes, x, y, u, v, density=1, linewidth=None, color=None,
arrows.append(p)
lc = mcollections.LineCollection(
- streamlines, transform=transform, **line_kw)
+ streamlines, transform=transform, **line_kw, capstyle='round')
lc.sticky_edges.x[:] = [grid.x_origin, grid.x_origin + grid.width]
lc.sticky_edges.y[:] = [grid.y_origin, grid.y_origin + grid.height]
if use_multicolor_lines:

python scatter plot with errorbars and colors mapping a physical quantity

I'm trying to do a quite simple scatter plot with error bars and semilogy scale. What is a little bit different from tutorials I have found is that the color of the scatterplot should trace a different quantity. On one hand, I was able to do a scatterplot with the errorbars with my data, but just with one color. On the other hand, I realized a scatterplot with the right colors, but without the errorbars.
I'm not able to combine the two different things.
Here an example using fake data:
#!/usr/bin/env python
# -*- coding: utf-8 -*-
from __future__ import division
import numpy as np
import matplotlib.pyplot as plt
n=100
Lx_gas = 1e40*np.random.random(n) + 1e37
Tx_gas = np.random.random(n) + 0.5
Lx_plus_error = Lx_gas
Tx_plus_error = Tx_gas/2.
Tx_minus_error = Tx_gas/4.
#actually positive numbers, this is the quantity that should be traced by the
#color, in this example I use random numbers
Lambda = np.random.random(n)
#this is actually different from zero, but I want to be sure that this simple
#code works with the log axis
Lx_minus_error = np.zeros_like(Lx_gas)
#normalize the color, to be between 0 and 1
colors = np.asarray(Lambda)
colors -= colors.min()
colors *= (1./colors.max())
#build the error arrays
Lx_error = [Lx_minus_error, Lx_plus_error]
Tx_error = [Tx_minus_error, Tx_plus_error]
##--------------
##important part of the script
##this works, but all the dots are of the same color
#plt.errorbar(Tx_gas, Lx_gas, xerr = Tx_error,yerr = Lx_error,fmt='o')
##this is what is should be in terms of colors, but it is without the error bars
#plt.scatter(Tx_gas, Lx_gas, marker='s', c=colors)
##what I tried (and failed)
plt.errorbar(Tx_gas, Lx_gas, xerr = Tx_error,yerr = Lx_error,\
color=colors, fmt='o')
ax = plt.gca()
ax.set_yscale('log')
plt.show()
I even tried to plot the scatterplot after the errorbar, but for some reason everything plotted on the same window is put in background with respect to the errorplot.
Any ideas?
Thanks!

You can set the color to the LineCollection object returned by the errorbar as described here.
from __future__ import division
import numpy as np
import matplotlib.pyplot as plt
n=100
Lx_gas = 1e40*np.random.random(n) + 1e37
Tx_gas = np.random.random(n) + 0.5
Lx_plus_error = Lx_gas
Tx_plus_error = Tx_gas/2.
Tx_minus_error = Tx_gas/4.
#actually positive numbers, this is the quantity that should be traced by the
#color, in this example I use random numbers
Lambda = np.random.random(n)
#this is actually different from zero, but I want to be sure that this simple
#code works with the log axis
Lx_minus_error = np.zeros_like(Lx_gas)
#normalize the color, to be between 0 and 1
colors = np.asarray(Lambda)
colors -= colors.min()
colors *= (1./colors.max())
#build the error arrays
Lx_error = [Lx_minus_error, Lx_plus_error]
Tx_error = [Tx_minus_error, Tx_plus_error]
sct = plt.scatter(Tx_gas, Lx_gas, marker='s', c=colors)
cb = plt.colorbar(sct)
_, __ , errorlinecollection = plt.errorbar(Tx_gas, Lx_gas, xerr = Tx_error,yerr = Lx_error, marker = '', ls = '', zorder = 0)
error_color = sct.to_rgba(colors)
errorlinecollection[0].set_color(error_color)
errorlinecollection[1].set_color(error_color)
ax = plt.gca()
ax.set_yscale('log')
plt.show()

producing histogram with y axis as relative frequency?

Today my task is to produce a histogram where the y axis is a relative frequency rather than just an absolute count. I've located another question regarding this (see: Setting a relative frequency in a matplotlib histogram) however, when I try to implement it, I get the error message:
'list' object has no attribute size
despite having the exact same code given in the answer -- and despite their information also being stored in a list.
In addition, I have tried the method here(http://www.bertplot.com/visualization/?p=229) with no avail, as the output still doesn't show the y label as ranging from 0 to 1.
import numpy as np
import matplotlib.pyplot as plt
import random
from tabulate import tabulate
import matplotlib.mlab as mlab
precision = 100000000000
def MarkovChain(n,s) :
"""
"""
matrix = []
for l in range(n) :
lineLst = []
sum = 0
crtPrec = precision
for i in range(n-1) :
val = random.randrange(crtPrec)
sum += val
lineLst.append(float(val)/precision)
crtPrec -= val
lineLst.append(float(precision - sum)/precision)
matrix2 = matrix.append(lineLst)
print("The intial probability matrix.")
print(tabulate(matrix2))
baseprob = []
baseprob2 = []
baseprob3 = []
baseprob4 = []
for i in range(1,s): #changed to do a range 1-s instead of 1000
#must use the loop variable here, not s (s is always the same)
matrix_n = np.linalg.matrix_power(matrix2, i)
baseprob.append(matrix_n.item(0))
baseprob2.append(matrix_n.item(1))
baseprob3.append(matrix_n.item(2))
baseprob = np.array(baseprob)
baseprob2 = np.array(baseprob2)
baseprob3 = np.array(baseprob3)
baseprob4 = np.array(baseprob4)
# Here I tried to make a histogram using the plt.hist() command, but the normed=True doesn't work like I assumed it would.
'''
plt.hist(baseprob, bins=20, normed=True)
plt.show()
'''
#Here I tried to make a histogram using the method from the second link in my post.
# The code runs, but then the graph that is outputted isn't doesn't have the relative frequency on the y axis.
'''
n, bins, patches = plt.hist(baseprob, bins=30,normed=True,facecolor = "green",)
y = mlab.normpdf(bins,mu,sigma)
plt.plot(bins,y,'b-')
plt.title('Main Plot Title',fontsize=25,horizontalalignment='right')
plt.ylabel('Count',fontsize=20)
plt.yticks(fontsize=15)
plt.xlabel('X Axis Label',fontsize=20)
plt.xticks(fontsize=15)
plt.show()
'''
# Here I tried to make a histogram using the method seen in the Stackoverflow question I mentioned.
# The figure that pops out looks correct in terms of the axes, but no actual data is posted. Instead the error below is shown in the console.
# AttributeError: 'list' object has no attribute 'size'
fig = plt.figure()
ax = fig.add_subplot(111)
ax.hist(baseprob, weights=np.zeros_like(baseprob)+1./ baseprob.size)
n, bins, patches = ax.hist(baseprob, bins=100, normed=1, cumulative=0)
ax.set_xlabel('Bins', size=20)
ax.set_ylabel('Frequency', size=20)
ax.legend
plt.show()
print("The final probability matrix.")
print(tabulate(matrix_n))
matrixTranspose = zip(*matrix_n)
evectors = np.linalg.eig(matrixTranspose)[1][:,0]
print("The steady state vector is:")
print(evectors)
MarkovChain(5, 1000)
The methods I tried are each commented out, so to reproduce my errors, make sure to erase the comment markers.
As you can tell, I'm really new to Programming. Also this is not for a homework assignment in a computer science class, so there are no moral issues associated with just providing me with code.

The expected input to matplotlib functions are usually numpy arrays, which have the methods nparray.size. Lists do not have size methods so when list.size is called in the hist function, this causes your error. You need to convert, using nparray = np.array(list). You can do this after the loop where you build the lists with append, something like,
baseprob = []
baseprob2 = []
baseprob3 = []
baseprob4 = []
for i in range(1,s): #changed to do a range 1-s instead of 1000
#must use the loop variable here, not s (s is always the same)
matrix_n = numpy.linalg.matrix_power(matrix, i)
baseprob.append(matrix_n.item(0))
baseprob2.append(matrix_n.item(1))
baseprob3.append(matrix_n.item(2))
baseprob = np.array(baseprob)
baseprob2 = np.array(baseprob2)
baseprob3 = np.array(baseprob3)
baseprob4 = np.array(baseprob4)
EDIT: minimal hist example
import numpy as np
import matplotlib.pyplot as plt
fig = plt.figure()
ax = fig.add_subplot(111)
baseprob = np.random.randn(1000000)
ax.hist(baseprob, weights=np.zeros_like(baseprob)+1./ baseprob.size, bins=100)
n, bins, patches = ax.hist(baseprob, bins=100, normed=1, cumulative=0, alpha = 0.4)
ax.set_xlabel('Bins', size=20)
ax.set_ylabel('Frequency', size=20)
ax.legend
plt.show()
which gives,

Creating a hexagonal grid (u-matrix) in Python using a Regularpolycollection

I am trying to create a hexagonal grid to use with a u-matrix in Python (3.4) using a RegularPolyCollection (see code below) and have run into two problems:
The hexagonal grid is not tight. When I plot it there are empty spaces between the hexagons. I can fix this by resizing the window, but since this is not reproducible and I want all of my plots to have the same size, this is not satisfactory. But even if it were, I run into the second problem.
Either the top or right hexagons don't fit in the figure and are cropped.
I have tried a lot of things (changing figure size, subplot_adjust(), different areas, different values of d, etc.) and I am starting to get crazy! It feels like the solution should be simple, but I simply cannot find it!
import SOM
import matplotlib.pyplot as plt
from matplotlib.collections import RegularPolyCollection
import numpy as np
import matplotlib.cm as cm
from mpl_toolkits.axes_grid1 import make_axes_locatable
m = 3 # The height
n = 3 # The width
# Some maths regarding hexagon geometry
d = 10
s = d/(2*np.cos(np.pi/3))
h = s*(1+2*np.sin(np.pi/3))
r = d/2
area = 3*np.sqrt(3)*s**2/2
# The center coordinates of the hexagons are calculated.
x1 = np.array([d*x for x in range(2*n-1)])
x2 = x1 + r
x3 = x2 + r
y = np.array([h*x for x in range(2*m-1)])
c = []
for i in range(2*m-1):
if i%4 == 0:
c += [[x,y[i]] for x in x1]
if (i-1)%2 == 0:
c += [[x,y[i]] for x in x2]
if (i-2)%4 == 0:
c += [[x,y[i]] for x in x3]
c = np.array(c)
# The color of the hexagons
d_matrix = np.zeros(3*3)
# Creating the figure
fig = plt.figure(figsize=(5, 5), dpi=100)
ax = fig.add_subplot(111)
# The collection
coll = RegularPolyCollection(
numsides=6, # a hexagon
rotation=0,
sizes=(area,),
edgecolors = (0, 0, 0, 1),
array= d_matrix,
cmap = cm.gray_r,
offsets = c,
transOffset = ax.transData,
)
ax.add_collection(coll, autolim=True)
ax.axis('off')
ax.autoscale_view()
plt.show()

See this topic
Also you need to add scale on axis like
ax.axis([xmin, xmax, ymin, ymax])

The hexalattice module of python (pip install hexalattice) gives solution to both you concerns:
Grid tightness: You have full control over the hexagon border gap via the 'plotting_gap' argument.
The grid plotting takes into account the grid final size, and adds sufficient margins to avoid the crop.
Here is a code example that demonstrates the control of the gap, and correctly fits the grid into the plotting window:
from hexalattice.hexalattice import *
create_hex_grid(nx=5, ny=5, do_plot=True) # Create 5x5 grid with no gaps
create_hex_grid(nx=5, ny=5, do_plot=True, plotting_gap=0.2)
See this answer for additional usage examples, more images and links
Disclosure: the hexalattice module was written by me