updated my fork from the upstream master
added display() and color() getter/setters to Axis to allow easier control over appearance of an Axis
Merge pull request #68 from Stavrakakis/master

bugfixes, typos and new Axis display functionality
tidied up how stacked/grouped series refer to the current series being drawn in the group (series.currentSeries)

fixed reference to self instead of this that didn't work
renamed loop variable to make it clear it's an index
Merge pull request #69 from Stavrakakis/master

tidied up how stacked/grouped series refer to the current series being d...
Fix gridlines for charts

Adds the gridlines back in, horizontally and vertically. Gridlines are
now clipped to be within the plot area of the chart, and line up with
the tick marks.

Also modified the gridlines() setter function to update whether or not
gridlines are visible for the chart.