Bug in plotting routine for negative sigmoids

[guillermoaguilar]

Plotting functions get the y-range of the sigmoid wrong.

Minimal example code:

import matplotlib.pyplot as plt
import psignifit as ps
import psignifit.psigniplot as psp

data = [[0.15, 10, 10], [0.16, 9, 10], [0.17, 8, 10], [0.18, 6, 10], [0.185, 5, 10]]
res = ps.psignifit(data, sigmoid="neg_norm", experiment_type="2AFC")
fig = plt.figure()  
res_plot = psp.plot_psychometric_function(res)

plt.show()
plt.close()

produces