create margins with constant size

With this we will be able to zoom in by selecting a region. The constant
margins allow us to determine the exact timestampt for a pixel position.

pdb-plotting/src/main/java/org/lucares/recommind/logs/GnuplotFileGenerator.java

@@ -76,6 +76,14 @@ public class GnuplotFileGenerator {
 			appendln(result, "set format y2 \"\"");
 
 			appendln(result, "set nokey");
+		} else if (!settings.isKeyOutside()) {
+			// make sure left and right margins are always the same
+			// this is need to be able to zoom in by selecting a region
+			// (horizontal: 1 unit = 10px; vertical: 1 unit = 19px)
+			appendln(result, "set lmargin 11"); // margin 11 -> 110px
+			appendln(result, "set rmargin 11"); // margin 11 -> 110px
+			appendln(result, "set tmargin 3"); // margin 3 -> 57px
+			appendln(result, "set bmargin 4"); // margin 4 -> 76
 		}
 
 		appendf(result, "plot ");