Barplot: Specific width for each bar

Regarding: https://github.com/flot/flot/pull/25

Regarding the comment posted there:
* It shouldn't assume that the new attribute is the last one anymore. But it does assume that the new attribute is the fourth one.
* I'm not sure this third coordinate is actually necessary. What does it do? Currently it seems that it's just a placeholder for 0. 
* Pointformat has seemed to come later than the previous pull request. Added that in this impl.

I'm not sure this is the most elegant approach. But, yes, now the xmin, xmax, ymin and ymax have to be checked for each point. Because any of them can be one of those values with the new dynamic bar width.

Also created an example page, where different plots can be tested out.