add capability for horizontal bar charting

[tomkralidis]

From email discussion:

> Hi Thomas,
>
> I was wondering whether it would be useful to have bar charts 
> available to be drawn horizontally in addition to (the "normal") 
> vertically.  I've seen examples of this and wonder if this could be 
> incorporated in mapchart.c.

it shouldn't be very complicated to add that

>
> From a user point of view, the default bar chart behaviour would be 
> vertical (as it is now).  We would add an optional PROCESSING 
> directive called "CHART_DIRECTION=foo", where foo is either x or y.  
> If "CHART_DIRECTION=x", then mapchart.c would rotate the chart 90 
> degrees clockwise.

or maybe another type of chart (CHART_TYPE=....) , there's already been a request for stacked bars, and we could also image line ones

[tbonfort]

implemented in 5.6 ( see #3128 )