This is meant to address #2056, where the vertical axis collapses when a list of large, constant values is plotted.

The reason for this is that the default 1.0 span of the auto-scaling eventually becomes smaller than the effective (~3e-16) resolution of a double float, and the range calculations break down.

This PR sets the minimum displayed range to 6E-15 of the center value, which makes sure that at least 20 quantization steps are on screen. At this level, the errors become obvious, but the plot remains visible, and the axis manages to calculate ticks and labels.

For values < 1.6e+14, the behavior should be unchanged, except that there is now a hard lower limit on the plot range: It is no longer possible to deliberately collapse the axis range to zero. However, I could not come up with a good reason to do so.

Here is some code to play around with, based on the report in #2056:

from pyqtgraph.Qt import QtGui
import pyqtgraph as pg

app = pg.mkQApp()
mw = QtGui.QMainWindow()
cw = QtGui.QWidget()
mw.setCentralWidget(cw)
l = QtGui.QVBoxLayout()
cw.setLayout(l)

pw = pg.PlotWidget(name='Plot1') 
l.addWidget(pw)
mw.show()
constant = 8e15 # now keeps working until other things break for >8e130 
p1 = pw.plot(y=[constant,constant])

# uncomment this to see quantization errors:
# p1 = pw.plot(y=[constant,constant+1,constant+2])

# uncomment this to see hard limit on axis range:
# pw.setYRange(constant-0, constant+0, padding=0)

if __name__ == '__main__':
    pg.exec()

Closes #2056, I hope.