Warn if points are out of range

Some users may improperly define the range of their axes, leading to
points being drawn outside of the chart area. In the worst cases, if
all points are out of range, the chart will not be drawn at all.

Adding a warning can help users identify this issue and correct it.

Note that may spew a lot of warnings if their are a lot of points.

src/dygraph-layout.js

@@ -253,6 +253,9 @@ DygraphLayout.prototype._evaluateLineCharts = function() {
 
       // Range from 0-1 where 0 represents left and 1 represents right.
       point.x = DygraphLayout.calcXNormal_(point.xval, this._xAxis, isLogscaleForX);
+      if (point.x < 0 || point.x > 1) {
+        console.warn('x value ' + point.xval + ' out of range ' + this._xAxis.minval + ' - ' + this._xAxis.maxval);
+      }
       // Range from 0-1 where 0 represents top and 1 represents bottom
       var yval = point.yval;
       if (isStacked) {
@@ -269,6 +272,9 @@ DygraphLayout.prototype._evaluateLineCharts = function() {
         }
       }
       point.y = DygraphLayout.calcYNormal_(axis, yval, logscale);
+      if (point.y < 0 || point.y > 1) {
+        console.warn('y value ' + point.yval + ' out of range ' + axis.minyval + ' - ' + axis.maxyval);
+      }
     }
 
     this.dygraph_.dataHandler_.onLineEvaluated(points, axis, logscale);