add label line clip pad

... that is add 2 px around labels text clip out of contour lines below.

    This becomes especially useful when for negative contour labels,
    to differentiate between the minus sign and the contour line

Author: etpinard

src/traces/contour/constants.js

@@ -72,5 +72,8 @@ module.exports = {
         INITIALSEARCHPOINTS: 10,
         // number of binary search iterations after the initial wide search
         ITERATIONS: 5
-    }
+    },
+
+    // number of px around labels text clip out of contour lines below
+    LABEL_LINE_CLIP_PAD: 2
 };

src/traces/contour/plot.js

@@ -26,6 +26,7 @@ var convertToConstraints = require('./convert_to_constraints');
 var closeBoundaries = require('./close_boundaries');
 var constants = require('./constants');
 var costConstants = constants.LABELOPTIMIZER;
+var LABEL_LINE_CLIP_PAD = constants.LABEL_LINE_CLIP_PAD;
 
 exports.plot = function plot(gd, plotinfo, cdcontours, contourLayer) {
     var xa = plotinfo.xaxis;
@@ -543,8 +544,8 @@ function locationCost(loc, textOpts, labelData, bounds) {
 }
 
 exports.addLabelData = function(loc, textOpts, labelData, labelClipPathData) {
-    var halfWidth = textOpts.width / 2;
-    var halfHeight = textOpts.height / 2;
+    var halfWidth = textOpts.width / 2 + LABEL_LINE_CLIP_PAD;
+    var halfHeight = textOpts.height / 2 + LABEL_LINE_CLIP_PAD;
 
     var x = loc.x;
     var y = loc.y;