Add datasetLabel to BarClass object

The datasetLabel attribute exists in the regular Bar but is missing from StackedBar.
Merge pull request #1 from muneeb456/patch-1

Add datasetLabel to BarClass object
Correct bar width for stacked bar

Since a stacked bar is displayed as a single vertical bar with segments representing the different datasets, the width of each bar corresponds to the entire bar width available and does not depend on the number of datasets. The x-coordinate of the bar is at the beginning of the section available for that bar.

Currently, with multiple datasets, the bar is not given its full width.
Merge pull request #2 from muneeb456/patch-1

Correct bar width for stacked bar