/GMARKER, CURVE, KEY, INCR
Specifies the curve marking style.
CURVECurve number markers will be applied on (integer value between 1 and 10).
KEYMarker key:
0  | —  | No markers will be applied (default).  | 
1  | —  | TRIANGLES will be applied.  | 
2  | —  | SQUARES will be applied.  | 
3  | —  | DIAMONDS will be applied.  | 
4  | —  | CROSSES will be applied.  | 
INCRDetermines the curve marking frequency. (a whole number value
between 1 and 255). If INCR = 1, markers are displayed
at every data point on the curve. If INCR = 2 then
markers are displayed at every second data point. If INCR =
3 then they are displayed at every third data point. 
The user-specified markers will not be drawn when the area under the curve is color-filled (/GROPT, FILL).