Better control of error bars from Python

[NickDraper]

From qtiplot handbook:

Error Bars

Having a plot layer l, you can add error bars to a data curve c, named curveName, using the following methods:

err1 = l.addErrorBars(c, Table *t, QString errColName, int type = 1, double width = 1, int capLength = 8, color = Qt.black, throughSymbol = True, minusSide = True, plusSide = True)
err2 = l.addErrorBars(curveName, Table *t, QString errColName, int type = 1, double width = 1, int capLength = 8, color = Qt.black, throughSymbol = True, minusSide = True, plusSide = True)
Each data curve, c, can have attached a list of error bars:

errors = c.errorBarsList()
The properties of an error bar curve can be accesses, via the following methods:

err = c.errorBarsList()[0]
for i in range(0, err.dataSize()):
    print err.errorValue(i)

err.capLength()
err.width()
err.color()
err.direction()
err.xErrors()
err.throughSymbol()
err.plusSide()
err.minusSide()
c = err.masterCurve() # reference to the master curve to which the error bars curve is attached.
err.detachFromMasterCurve() # equivalent to c.removeErrorBars(err)
... and can be modified, via the following methods:

err.setCapLength(12)
err.setWidth(3)
err.setColor(Qt.red)
err.setDirection(ErrorBarsCurve.Vertical)
err.setXErrors(True) # equivalent to err.setDirection(ErrorBarsCurve.Horizontal)
err.drawThroughSymbol(True)
err.drawPlusSide(True)
err.drawMinusSide(False)
err.setMasterCurve(c)
You can remove all error bars attached to a curve using:

c.clearErrorBars()

[NickDraper]

This issue was originally trac ticket 2657