unexposing contextcolor; changing default opacity

(cherry picked from commit 01ba2da)

Author: monfera

src/traces/parcoords/attributes.js

@@ -156,16 +156,9 @@ module.exports = {
     },
 
     lines: {
-        contextcolor: {
-            valType: 'data_array',
-            dflt: [0, 0, 0],
-            role: 'style',
-            description: 'Color of the context line layer as an RGB triplet where each number is 0..255.'
-        },
-
         contextopacity: {
             valType: 'number',
-            dflt: 0.025,
+            dflt: 0.05,
             min: 0,
             max: 1,
             role: 'style',

src/traces/parcoords/defaults.js

@@ -85,7 +85,6 @@ module.exports = function supplyDefaults(traceIn, traceOut, defaultColor, layout
 
     coerce('line.color');
 
-    coerce('lines.contextcolor');
     coerce('lines.contextopacity');
     coerce('lines.pixelratio');
     coerce('lines.focusalphablending');

src/traces/parcoords/lines.js

@@ -20,6 +20,8 @@ var gpuDimensionCount = 64;
 var sectionVertexCount = 2;
 var vec4NumberCount = 4;
 
+var contextColor = [119, 119, 119]; // middle gray to not drawn the focus; looks good on a black or white background
+
 var dummyPixel = new Uint8Array(4);
 function ensureDraw(regl) {
     regl.read({
@@ -87,11 +89,11 @@ function adjustDepth(d) {
     return Math.max(depthLimitEpsilon, Math.min(1 - depthLimitEpsilon, d));
 }
 
-function palette(unitToColor, context, lines_contextcolor, contextOpacity) {
+function palette(unitToColor, context, contextOpacity) {
     var result = [];
     for(var j = 0; j < 256; j++) {
         var c = unitToColor(j / 255);
-        result.push((context ? lines_contextcolor : c).concat([context ? Math.round(contextOpacity * 255) : 255]));
+        result.push((context ? contextColor : c).concat([context ? Math.round(contextOpacity * 255) : 255]));
     }
 
     return result;
@@ -181,7 +183,7 @@ module.exports = function(canvasGL, lines, canvasWidth, canvasHeight, dimensions
         type: 'uint8',
         mag: 'nearest',
         min: 'nearest',
-        data: palette(unitToColor, context, lines.contextcolor, lines.contextopacity)
+        data: palette(unitToColor, context, lines.contextopacity)
     });
 
     var glAes = regl({