[matplotlib] pyplot scatter plot marker size

Because other answers here claim that s denotes the area of the marker, I'm adding this answer to clearify that this is not necessarily the case.

Size in points^2

The argument s in plt.scatter denotes the markersize**2. As the documentation says

s : scalar or array_like, shape (n, ), optional
size in points^2. Default is rcParams['lines.markersize'] ** 2.

This can be taken literally. In order to obtain a marker which is x points large, you need to square that number and give it to the s argument.

So the relationship between the markersize of a line plot and the scatter size argument is the square. In order to produce a scatter marker of the same size as a plot marker of size 10 points you would hence call scatter( .., s=100).

import matplotlib.pyplot as plt

fig,ax = plt.subplots()

ax.plot([0],[0], marker="o",  markersize=10)
ax.plot([0.07,0.93],[0,0],    linewidth=10)
ax.scatter([1],[0],           s=100)

ax.plot([0],[1], marker="o",  markersize=22)
ax.plot([0.14,0.86],[1,1],    linewidth=22)
ax.scatter([1],[1],           s=22**2)

plt.show()

Connection to "area"

So why do other answers and even the documentation speak about "area" when it comes to the s parameter?

Of course the units of points**2 are area units.

• For the special case of a square marker, marker="s", the area of the marker is indeed directly the value of the s parameter.
• For a circle, the area of the circle is area = pi/4*s.
• For other markers there may not even be any obvious relation to the area of the marker.

In all cases however the area of the marker is proportional to the s parameter. This is the motivation to call it "area" even though in most cases it isn't really.

Specifying the size of the scatter markers in terms of some quantity which is proportional to the area of the marker makes in thus far sense as it is the area of the marker that is perceived when comparing different patches rather than its side length or diameter. I.e. doubling the underlying quantity should double the area of the marker.

What are points?

So far the answer to what the size of a scatter marker means is given in units of points. Points are often used in typography, where fonts are specified in points. Also linewidths is often specified in points. The standard size of points in matplotlib is 72 points per inch (ppi) - 1 point is hence 1/72 inches.

It might be useful to be able to specify sizes in pixels instead of points. If the figure dpi is 72 as well, one point is one pixel. If the figure dpi is different (matplotlib default is fig.dpi=100),

1 point == fig.dpi/72. pixels

While the scatter marker's size in points would hence look different for different figure dpi, one could produce a 10 by 10 pixels^2 marker, which would always have the same number of pixels covered:

import matplotlib.pyplot as plt

for dpi in [72,100,144]:

    fig,ax = plt.subplots(figsize=(1.5,2), dpi=dpi)
    ax.set_title("fig.dpi={}".format(dpi))

    ax.set_ylim(-3,3)
    ax.set_xlim(-2,2)

    ax.scatter([0],[1], s=10**2, 
               marker="s", linewidth=0, label="100 points^2")
    ax.scatter([1],[1], s=(10*72./fig.dpi)**2, 
               marker="s", linewidth=0, label="100 pixels^2")

    ax.legend(loc=8,framealpha=1, fontsize=8)

    fig.savefig("fig{}.png".format(dpi), bbox_inches="tight")

plt.show() 

If you are interested in a scatter in data units, check this answer.