Patch for Issue 699 and 628 & allows area without lines.

- Fixes the issue where steps altering user datapoints
- Do not altering user data points for drawing lines with steps 
(even stacked); just consider drawing with steps enables a different 
kind of plotting and clipping on lines and areas; that assuming that 
drawing with steps must always go horizontally and then verticaly 
(like previous inserted datapoints).
- Allow to draw lines:fill without lines:show that able to display
 area charts