Fix trend line writing over axis.

Fixes #21

src/chartjs-plugin-trendline.js

@@ -58,6 +58,27 @@ function addFitter(datasetMeta, ctx, dataset, xScale, yScale) {
     var y1 = yScale.getPixelForValue(fitter.f(fitter.minx));
     var y2 = yScale.getPixelForValue(fitter.f(fitter.maxx));
     if ( !xy ) { x1 = startPos; x2 = endPos; }
+
+    const drawBottom = datasetMeta.controller.chart.chartArea.bottom;
+    const chartWidth = datasetMeta.controller.chart.width;
+
+    if(y1 > drawBottom) { // Left side is below zero
+        const diff = y1 - drawBottom;
+        const lineHeight = y1 - y2;
+        const overlapPercentage = diff / lineHeight;
+        const addition = chartWidth * overlapPercentage;
+
+        y1 = drawBottom;
+        x1 = (x1 + addition);
+    } else if(y2 > drawBottom) { // right side is below zero
+        const diff = y2 - drawBottom;
+        const lineHeight = y2 - y1;
+        const overlapPercentage = diff / lineHeight;
+        const subtraction = chartWidth - (chartWidth * overlapPercentage);
+
+        y2 = drawBottom;
+        x2 = chartWidth - (x2 - subtraction);
+    }
 
     ctx.lineWidth = lineWidth;
     if (lineStyle === "dotted") { ctx.setLineDash([2, 3]); }
@@ -96,4 +117,4 @@ LineFitter.prototype = {
         var scale = (this.count * this.sumXY - this.sumX * this.sumY) / det;
         return offset + x * scale;
     }
-};
+};