Removing Data Below A Line In A Scatterplot (Python)

Question

So I had code that graphed a 2dhistogram of my dataset. I plotted it like so:

histogram = plt.hist2d(fehsc, ofesc, bins=nbins, range=[[-1,.5],[0.225,0.4]])
         


        

Answer 1

You could define a mask for your data before you plot and then just plot the data points that actually meet your criteria. Below an example, where all data points above a certain line are plotted in green and all data points below the line are plotted in black.

from matplotlib import pyplot as plt
import numpy as np

#the scatterplot data
xvals = np.random.rand(100)
yvals = np.random.rand(100)

#the line
b  = 0.1
m = 1
x = np.linspace(0,1,num=100)
y = m*x+b

mask = yvals > m*xvals+b

plt.scatter(xvals[mask],yvals[mask],color='g')
plt.scatter(xvals[~mask],yvals[~mask],color='k')
plt.plot(x,y,'r')
plt.show()

The result looks like this

Hope this helps.

EDIT:

If you want to create a 2D histogram, where the portion below the line is set to zero, you can do that by first generating the histogram using numpy (as an array) and then setting the values inside that array to zero, if the bins fall below the line. After that, you can plot the matrix using plt.pcolormesh:

from matplotlib import pyplot as plt
import numpy as np

#the scatterplot data
xvals = np.random.rand(1000)
yvals = np.random.rand(1000)
histogram,xbins,ybins = np.histogram2d(xvals,yvals,bins=50)

#computing the bin centers from the bin edges:
xcenters = 0.5*(xbins[:-1]+xbins[1:])
ycenters = 0.5*(ybins[:-1]+ybins[1:])

#the line
b  = 0.1
m = 1
x = np.linspace(0,1,num=100)
y = m*x+b

#hiding the part of the histogram below the line
xmesh,ymesh = np.meshgrid(xcenters,ycenters)
mask = m*xmesh+b > ymesh
histogram[mask] = 0

#making the plot
mat = plt.pcolormesh(xcenters,ycenters,histogram)
line = plt.plot(x,y,'r')
plt.xlim([0,1])
plt.ylim([0,1])
plt.show()

The result would be something like this: