Using matplotlib, how can I print something "actual size"?

Question

I have a number of plots where the x- and y-axes are in centimeter units, and I am already using axis('equal') to ensure proper aspect ratios. I would like to print out those plots so that when I measure distances within my axes, the distances correspond to real-world centimeters.

That is, a line that is 3 units (cm) long in the plot should print out to be 3 cm long. (A more complex example would be drawing a ruler in Matplotlib, and then printing it out for use /as/ a ruler.) I found solutions in matlab and mathematica, but not for Matplotlib. Is there a magic formula to achieve this? I believe it will require a special combination/ordering of: figure(figsize=??), axis('equal'), fig.canvas.draw(), fig.savefig('filename',format="??"), possibly some math with fig.bbox parameters, and perhaps one or more dpi-settings. I have tried many combinations but haven't hit on the right one. And maybe there is a way-easier approach ...

Answer 1

Consider this example. Where I specify exactly the dimension of my axes in cm. matplotlib works in inches, so I convert to inches. And then I also save it with a particular dpi (128) so that it matches the designed dimensions in my display. This of course varies for every display. I found that by trial and error, even though there might be other methods. Well here the code:

left_margin = 1.   # cm
right_margin = 1.  # cm
figure_width = 10. # cm
figure_height = 7. # cm
top_margin = 1.    # cm
bottom_margin = 1. # cm

box_width = left_margin + figure_width + right_margin   # cm
box_height = top_margin + figure_height + bottom_margin # cm

cm2inch = 1/2.54 # inch per cm

# specifying the width and the height of the box in inches
fig = figure(figsize=(box_width*cm2inch,box_height*cm2inch))
ax = fig.add_subplot(111)
ax.plot([1,2,3])

fig.subplots_adjust(left   = left_margin / box_width,
                    bottom = bottom_margin / box_height,
                    right  = 1. - right_margin / box_width,
                    top    = 1. - top_margin   / box_height,
                    )
fig.savefig('ten_x_seven_cm.png', dpi=128)
# dpi = 128 is what works in my display for matching the designed dimensions.

Answer 2

Add this to @pablo reyes' answer, check that the printer is at 100%, and it's pretty close;

ax.set_ylim(0,7)
ax.set_xlim(0,10)

ax.plot([0.5, 1.5],[0.25, 0.25],label='One cm?')
ax.plot([6,6],[1,2], label='One cm?')
ax.legend()

we force the axis to be a size we know, we make its data-transform match the real world, and we can "print a ruler".