chartjs extension update

Hi ITG,

As has been my want, I needed some further features in the chartjs extension so I thought I would make them and submit them back to you to distribute or ignore as you wish. As with any previous changes I assign all rights over the source to ITG.

The changes I have made are:

Upgraded to chartjs 2.7.2

Added new properties to the line chart dataset:

ShowLine – if false then line is not drawn between points – default: true
SteppedLine – Allows stepped lines to be drawn between points – default: false (i.e no steps).
SpanGaps – If true, lines will be drawn between points with no or null data. If false, points with NaN data will create a break in the line – Default: false.