Implement options to draw minor ticks and grid lines in Cartesian subplots

src/plots/cartesian/axis_defaults.js

@@ -4,6 +4,7 @@ var isNumeric = require('fast-isnumeric');
 
 var Registry = require('../../registry');
 var Lib = require('../../lib');
+var Template = require('../../plot_api/plot_template');
 
 var handleArrayContainerDefaults = require('../array_container_defaults');
 
@@ -121,16 +122,45 @@ module.exports = function handleAxisDefaults(containerIn, containerOut, coerce,
         color: dfltFontColor
     });
 
+    // major ticks
     handleTickValueDefaults(containerIn, containerOut, coerce, axType);
+
+    var hasMinor = options.hasMinor;
+    if(hasMinor) {
+        // minor ticks
+        Template.newContainer(containerOut, 'minor');
+        handleTickValueDefaults(containerIn, containerOut, coerce, axType, { isMinor: true });
+    }
+
     handleTickLabelDefaults(containerIn, containerOut, coerce, axType, options);
+
+    // major and minor ticks
     handleTickMarkDefaults(containerIn, containerOut, coerce, options);
+    if(hasMinor) {
+        var keepIsMinor = options.isMinor;
+        options.isMinor = true;
+        handleTickMarkDefaults(containerIn, containerOut, coerce, options);
+        options.isMinor = keepIsMinor;
+    }
+
     handleLineGridDefaults(containerIn, containerOut, coerce, {
         dfltColor: dfltColor,
         bgColor: options.bgColor,
         showGrid: options.showGrid,
+        hasMinor: hasMinor,
         attributes: layoutAttributes
     });
 
+    // delete minor when no minor ticks or gridlines
+    if(
+        hasMinor &&
+        !containerOut.minor.ticks &&
+        !containerOut.minor.showgrid
+    ) {
+        delete containerOut.minor;
+    }
+
+    // mirror
     if(containerOut.showline || containerOut.ticks) coerce('mirror');
 
     if(options.automargin) coerce('automargin');

src/plots/cartesian/index.js

@@ -461,6 +461,7 @@ function makeSubplotLayer(gd, plotinfo) {
             plotinfo.shapelayer = ensureSingle(backLayer, 'g', 'shapelayer');
             plotinfo.imagelayer = ensureSingle(backLayer, 'g', 'imagelayer');
 
+            plotinfo.minorGridlayer = ensureSingle(plotgroup, 'g', 'minor-gridlayer');
             plotinfo.gridlayer = ensureSingle(plotgroup, 'g', 'gridlayer');
             plotinfo.zerolinelayer = ensureSingle(plotgroup, 'g', 'zerolinelayer');
 
@@ -500,6 +501,7 @@ function makeSubplotLayer(gd, plotinfo) {
         // their other components to the corresponding
         // extra groups of their main plots.
 
+        plotinfo.minorGridlayer = mainplotinfo.minorGridlayer;
         plotinfo.gridlayer = mainplotinfo.gridlayer;
         plotinfo.zerolinelayer = mainplotinfo.zerolinelayer;
 
@@ -525,6 +527,12 @@ function makeSubplotLayer(gd, plotinfo) {
     // common attributes for all subplots, overlays or not
 
     if(!hasOnlyLargeSploms) {
+        ensureSingleAndAddDatum(plotinfo.minorGridlayer, 'g', plotinfo.xaxis._id);
+        ensureSingleAndAddDatum(plotinfo.minorGridlayer, 'g', plotinfo.yaxis._id);
+        plotinfo.minorGridlayer.selectAll('g')
+            .map(function(d) { return d[0]; })
+            .sort(axisIds.idSort);
+
         ensureSingleAndAddDatum(plotinfo.gridlayer, 'g', plotinfo.xaxis._id);
         ensureSingleAndAddDatum(plotinfo.gridlayer, 'g', plotinfo.yaxis._id);
         plotinfo.gridlayer.selectAll('g')