I was checking how ax.axvline does work, and I've written a small function that resembles part of its idea:
import matplotlib.pyplot as plt
import matplotlib.lines as mlines
def newline(p1, p2):
ax = plt.gca()
xmin, xmax = ax.get_xbound()
if(p2[0] == p1[0]):
xmin = xmax = p1[0]
ymin, ymax = ax.get_ybound()
else:
ymax = p1[1]+(p2[1]-p1[1])/(p2[0]-p1[0])*(xmax-p1[0])
ymin = p1[1]+(p2[1]-p1[1])/(p2[0]-p1[0])*(xmin-p1[0])
l = mlines.Line2D([xmin,xmax], [ymin,ymax])
ax.add_line(l)
return l
So, if you run the following code you will realize how does it work. The line will span the full range of your plot (independently on how big it is), and the creation of the line doesn't rely on any data point within the axis, but only in two fixed points that you need to specify.
import numpy as np
x = np.linspace(0,10)
y = x**2
p1 = [1,20]
p2 = [6,70]
plt.plot(x, y)
newline(p1,p2)
plt.show()
