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.

2 x points are allowed in order to draw the edges of the chart when
zoomed in.

src/dygraph-layout.js

@@ -247,12 +247,14 @@ DygraphLayout.prototype._evaluateLineCharts = function() {
     var axis = this.dygraph_.axisPropertiesForSeries(setName);
     // TODO (konigsberg): use optionsForAxis instead.
     var logscale = this.dygraph_.attributes_.getForSeries("logscale", setName);
+    var outOfXBounds = 0, outOfYBounds = 0;
 
     for (var j = 0; j < points.length; j++) {
       var point = points[j];
 
       // Range from 0-1 where 0 represents left and 1 represents right.
       point.x = DygraphLayout.calcXNormal_(point.xval, this._xAxis, isLogscaleForX);
+      outOfXBounds += (point.x < 0) || (point.x > 1);
       // Range from 0-1 where 0 represents top and 1 represents bottom
       var yval = point.yval;
       if (isStacked) {
@@ -269,6 +271,14 @@ DygraphLayout.prototype._evaluateLineCharts = function() {
         }
       }
       point.y = DygraphLayout.calcYNormal_(axis, yval, logscale);
+      outOfYBounds += (point.y < 0) || (point.y > 1);
+    }
+
+    if (outOfXBounds > 2) {
+      console.warn(outOfXBounds + ' points out of X bounds:' + this._xAxis.minval + ' - ' + this._xAxis.maxval);
+    }
+    if (outOfYBounds > 0) {
+      console.warn(outOfYBounds + ' points out of Y bounds:' + axis.minyval + ' - ' + axis.maxyval);
     }
 
     this.dygraph_.dataHandler_.onLineEvaluated(points, axis, logscale);