added support for 'less is worse' thresholds

Sometimes you want to express 'less is worse' thresholds. That means setting the error threshold below the warning threshold. The problem was, that the error marking always went up to +Infinity. Concerns 16599a07a9.

src/app/directives/grafanaGraph.js

@@ -141,7 +141,12 @@ function (angular, $, kbn, moment, _) {
             });
 
             if (panel.grid.threshold2) {
-              var limit2 = panel.grid.thresholdLine ? panel.grid.threshold2 : null;
+              var limit2;
+              if (panel.grid.thresholdLine) {
+                limit2 = panel.grid.threshold2;
+              } else {
+                limit2 = panel.grid.threshold1 > panel.grid.threshold2 ?  -Infinity : +Infinity;
+              }
               options.grid.markings.push({
                 yaxis: { from: panel.grid.threshold2, to: limit2 },
                 color: panel.grid.threshold2Color
@@ -340,4 +345,4 @@ function (angular, $, kbn, moment, _) {
     };
   });
 
-});
+});