#819 Optional vertical draw of error bars in legend keys

[Federico Ferri]

This patch introduces the possibility to have points in keys drawn with vertical error bars rather than horizontal error bars, via the key option Vertical. This is useful in scientific publications where it is often customary to indicate data with vertical errors bars also in the legends. The default vertical length is reduced by 1/4 of the sample length. In case a plot has xyerrorbars, if the Vertical option is present both the horizontal and the vertical error bars are drawn.
The patch is on top of 6.0.2. Minimal examples attached.