bar chart with a single datapoint is shown as empty.

When I try to render a bar chart with a single data point the bar is drawn with a height of zero because the min and max values are the same. This makes it look like the chart contains no data.

This patch allows rendering a bar chart with a single datapoint. This is achieved by defaulting to a minimum value of zero if only one datapoint is defined within a dataset.  The minimum value can be overriden by setting the scaleStartValue, in which case it is used as the minimum value for the bar.

My apologies for submitting the .gitgnore and ._DS_Store stuff in this pull request, GIT Hub hates me.