How to specify the margin of a plot in mm/cm using matplotlib?

Question

Example

Suppose I have two triangles:

• A triangle with points (0, 0), (10, 0), (10, 0.5) and
• a triangle with points (0, 0), (1, 0), (0.5, 11)

The resulting two plots without specifying the xlim and ylimlook like this:

Question

What do I need to do to satisfy all points listed below?

• Make the triangle visible, so that no line of the triangle is hidden by an axis
• Specify the same margin for all plots in mm, cm or other unit.
  (in the example above only two triangles were used. Actually I have n triangles.)
  As margin I mean the distance between the outer points of the triangle and the axis.

The resulting plots should look like this with the difference that the distances, which are marked with the red arrows, should all be the same!

Answer 1

Your two triangles with points (0, 0), (10, 0), (10, 0.5) and (0, 0), (1, 0), (0.5, 11) would be represented in pylab as:

Ax = [0, 10, 10]
Ay = [0, 0, 0.5]
Bx = [0, 1, 0.5]
By = [0, 0, 11]

pylab.plot(Ax, Ay)
pylab.plot(Bx, By)

Let's see what the lowest X value is:

lowestX = None

for x in Ax+Bx:
    if lowestX==None or x<lowestX:
        lowestX = x

Exercise for the reader to do the same for highestX, lowestY, and highestY.

Now, consider a boundary of 2 units, you can add / subtract these units from the lowest and highest values and set xlim and ylim:

margin = 2

pylab.xlim([lowestX-margin, highestX+margin])
pylab.ylim([lowestY-margin, highestY+margin])

Answer 2

I don't know of a way to to it in cm/mm, but you can do it with the precentage of the total size:

# you don't really need this see bellow
#from matplotlib.backends.backend_pdf import PdfPages
import pylab
import matplotlib.pyplot as plt

left,bottom,width,height = 0.2,0.1,0.6,0.6 # margins as % of canvas size 

fig = plt.figure(figsize=(4,4),facecolor="yellow") # figure size in Inches
fig.patch.set_alpha(0.8) # just a trick so you can see the difference 
                         # between the "canvas" and the axes
ax1 = plt.Axes(fig,[left,bottom,width,height])
ax1.plot([1,2,3,4],'b') # plot on the first axes you created
fig.add_axes(ax1)

ax1.plot([0,1,1,0,0], [0,0,1,1,0],"ro") # plot on the first axes you created
ax1.set_xlim([-1.1,2]) 
ax1.set_ylim([-1.1,2]) 

# pylab.plot([0,1,1,0,0], [0,0,1,1,0],"ro") avoid usig if you 
# want to control more frames in a plot
# see my answer here
#http://stackoverflow.com/questions/8176458/\
#remove-top-and-right-axis-in-matplotlib-after-\
#increasing-margins/8180844#8180844

# pdf = PdfPages("Test.pdf")# you don't really need this
# pylab.savefig(pdf, papertype = "a4", format = "pdf")
# automagically make your pdf like this
pylab.savefig("Test1.pdf", papertype="a4",facecolor='y')  
pylab.show()
pylab.close()       
# pdf.close()

and the output is:

corrected image: