[freechart] BUGFIX: H/V marker visibility check error
In some situations, the marker lines on an XY chart may become invisible. The attached patch fixes this issue.
In brief: Markers were not being checked against the correct axis. A horizontal marker line should be checked against the vertical (Y) axis (because it's plotted on that axis), while a vertical marker line should be checked against the horizontal (X) axis. This meant that if you had a Y axis value range of say 1..1000 (integer) and an X axis value range of 0.04 to 2.00 (float), any vertical markers located below value 1.0 would be ignored (half the value span).
Other code which calls Axis->IsVisible() should also be checked for correctness.