add a few comments

Author: etpinard

src/plots/cartesian/index.js

@@ -216,7 +216,7 @@ function plotOne(gd, plotinfo, cdSubplot, transitionOpts, makeOnCompleteCallback
             var className = (_module.layerName || name + 'layer');
             var plotMethod = _module.plot;
 
-            // plot all traces of this type on this subplot at once
+            // plot all visible traces of this type on this subplot at once
             cdModuleAndOthers = getModuleCalcData(cdSubplot, plotMethod);
             cdModule = cdModuleAndOthers[0];
             // don't need to search the found traces again - in fact we need to NOT

src/plots/get_data.js

@@ -69,6 +69,7 @@ exports.getModuleCalcData = function(calcdata, arg1) {
     for(var i = 0; i < calcdata.length; i++) {
         var cd = calcdata[i];
         var trace = cd[0].trace;
+        // N.B. 'legendonly' traces do not make it pass here
         if(trace.visible !== true) continue;
 
         // group calcdata trace not by 'module' (as the name of this function

src/traces/scattergl/index.js

@@ -572,6 +572,10 @@ function plot(gd, subplot, cdata) {
             }
 
             // precalculate px coords since we are not going to pan during select
+            // TODO, could do better here e.g.
+            // - spin that in a webworker
+            // - compute selection from polygons in data coordinates
+            //   (maybe just for linear axes)
             var xpx = stash.xpx = new Array(stash.count);
             var ypx = stash.ypx = new Array(stash.count);
             for(j = 0; j < stash.count; j++) {
@@ -614,6 +618,7 @@ function plot(gd, subplot, cdata) {
     // provide viewport and range
     var vpRange0 = {
         viewport: getViewport(fullLayout, xaxis, yaxis),
+        // TODO do we need those fallbacks?
         range: [
             (xaxis._rl || xaxis.range)[0],
             (yaxis._rl || yaxis.range)[0],