stepMode automatically draws left and right vertical lines

[pijyoi]

Just to document some inconsistencies in PlotCurveItem stepMode.
In stepMode, the vertical lines at the left and right edges are automatically added when fillLevel is active, differently from non-stepMode.
The code that does this is in PlotCurveItem::_generateStepModeData(). It has some overlap with what fillOutline=True does.

Top row is fillOutline=False
Bottom row is fillOutline=True

import pyqtgraph as pg 
import numpy as np 

yc = [7, 6, 5, 4, 3, 2, 3, 4, 5, 6, 7]
xc = np.arange(1, len(yc)+1)
xl = xc + 0.5
xr = xc - 0.5
xlr = np.arange(1, len(yc)+2) - 0.5

pg.mkQApp()
win = pg.GraphicsLayoutWidget(show=True)
plt1 = win.addPlot()
plt2 = win.addPlot()
plt3 = win.addPlot()
plt4 = win.addPlot()
win.nextRow()
plt5 = win.addPlot()
plt6 = win.addPlot()
plt7 = win.addPlot()
plt8 = win.addPlot()

pen = pg.mkPen(width=5)

fillopts = dict(pen=pen, fillLevel=0, fillOutline=False, brush='b')
plt1.plot(x=xl, y=yc, stepMode="left", **fillopts)
plt2.plot(x=xlr, y=yc, stepMode="center", **fillopts)
plt3.plot(x=xr, y=yc, stepMode="right", **fillopts)
plt4.plot(x=xc, y=yc, **fillopts)

fillopts = dict(pen=pen, fillLevel=0, fillOutline=True, brush='b')
plt5.plot(x=xl, y=yc, stepMode="left", **fillopts)
plt6.plot(x=xlr, y=yc, stepMode="center", **fillopts)
plt7.plot(x=xr, y=yc, stepMode="right", **fillopts)
plt8.plot(x=xc, y=yc, **fillopts)

pg.exec()

[pijyoi]

It appears that the code to have stepMode draw the left and right vertical lines when fillLevel is active has been there since the initial commit of PlotCurveItem.py.

Later on #922 and #999 add options to fully enclose the filled patch for other curves, but not taking into account that stepMode already partially encloses the patch (except for the final closing horizontal line)

[pijyoi]

Not sure that I would classify this as a bug. It does seem to make stepMode only useful for histograms.

Without this feature, stepMode could perhaps have been used for a digital plot.

In any case, all these niche extra options are confusing. While it's obvious that not all combinations are meaningful, it's not clear which ones are supported by pyqtgraph.

E.g. shadowPen will not draw the whole outline drawn by fillOutline