hiding line markers results in series labels being hidden

[nkammah]

I believe the culprit line is here : https://github.com/topfunky/gruff/blob/master/lib/gruff/base.rb#L541
I am just not sure if this is the intended behavior or not - the use case would be to generate bar charts without any line markers but with the series labels still, example :

[elrayle]

Partially addressed by PR #446 - adds support in bar charts only for hiding line markers and labels using separate variables.

[elrayle]

Partially addressed by:

• PR add hide_labels to stacked bar graphs #450 - add hide_labels to stacked bar graphs
• PR add hide_labels to side bar graphs #451 - add hide_labels to side bar graphs
• PR add hide_labels to stacked side bar graphs #452 - add hide_labels to stacked side bar graphs