Specify label for only one xValue of VLine

All xValues of a particular VLine use the same style. So we just need to
specify the label in the legend for one of the xValues.
Merge pull request #28 from sivasankariit/master

Option to set titlesize for plots
Merge pull request #29 from sivasankariit/vline_label_fix

Specify label for only one xValue of VLine