render images without any data on it instead of throwing errors

This makes it easier to use the mouse wheel
to zoom in. Without it you could zoom into
a region that had not data and then had to
use the date picker to change the date.

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

@@ -105,6 +105,10 @@ public class GnuplotFileGenerator {
 
 		settings.getAggregates().addPlots(result, dataSeries);
 		
+		// Add a plot outside of the visible range. Without this gnuplot would not
+		// render images when there are not data points on it.
+		appendf(result, "-1 with lines notitle");
+		
 		return result.toString();
 	}
 

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

@@ -97,10 +97,6 @@ public class Plotter {
       });
       METRICS_LOGGER.debug("csv generation took: " + (System.nanoTime() - start) / 1_000_000.0 + "ms");
 
-      if (dataSeries.isEmpty()) {
-        throw new NoDataPointsException();
-      }
-
       final Limit limitBy = plotSettings.getLimitBy();
       final int limit = plotSettings.getLimit();
       DataSeries.sortAndLimit(dataSeries, limitBy, limit);