Implemented a conveniance function xy-line-plot.

The treatment of line and color sequence may need to be reworked.
Cleaned up code comments
Modified lines, points, and lines-points to have cyclic propertes

Each subsequent call to lines, points, and lines-points will result
in a plot element having a different color and/or point style from the
previous element.
Added high-level functions xy-line-plot and xy-scatter-plot
Changed hard-coded color values to parameterized values
Cleaned up implementation of high-level plotting funs
Reverted lines, points, and lines-points to orig definition
Added style sequencing to xy-line-plot and xy-scatter-plot
Set legend (key) to be hidden by default