make a constant for font midline alignment

Author: alexcjohnson

src/constants/alignment.js

@@ -30,5 +30,13 @@ module.exports = {
         top: 0
     },
     // multiple of fontSize to get the vertical offset between lines
-    LINE_SPACING: 1.3
+    LINE_SPACING: 1.3,
+
+    // multiple of fontSize to shift from the baseline to the midline
+    // (to use when we don't calculate this shift from Drawing.bBox)
+    // To be precise this should be half the cap height (capital letter)
+    // of the font, and according to wikipedia:
+    //   an "average" font might have a cap height of 70% of the em
+    // https://en.wikipedia.org/wiki/Em_(typography)#History
+    MID_SHIFT: 0.35
 };

src/plots/cartesian/axes.js

@@ -28,6 +28,8 @@ var ONEHOUR = constants.ONEHOUR;
 var ONEMIN = constants.ONEMIN;
 var ONESEC = constants.ONESEC;
 
+var MID_SHIFT = require('../../constants/alignment').MID_SHIFT;
+
 var axes = module.exports = {};
 
 axes.layoutAttributes = require('./layout_attributes');
@@ -1796,7 +1798,7 @@ axes.doTicks = function(gd, axid, skipTitle) {
         else {
             flipit = (axside === 'right') ? 1 : -1;
             labely = function(d) {
-                return d.dy + d.fontSize * 0.35 - labelShift * flipit;
+                return d.dy + d.fontSize * MID_SHIFT - labelShift * flipit;
             };
             labelx = function(d) {
                 return d.dx + position + (labelStandoff + pad +

src/plots/polar/micropolar.js

@@ -9,6 +9,7 @@
 var d3 = require('d3');
 var Lib = require('../../lib');
 var extendDeepAll = Lib.extendDeepAll;
+var MID_SHIFT = require('../../constants/alignment').MID_SHIFT;
 
 var µ = module.exports = { version: '0.2.2' };
 
@@ -295,7 +296,7 @@ var µ = module.exports = { version: '0.2.2' };
             angularAxisEnter.append('text').classed('axis-text', true).style(fontStyle);
             var ticksText = angularAxis.select('text.axis-text').attr({
                 x: radius + axisConfig.labelOffset,
-                dy: '.35em',
+                dy: MID_SHIFT + 'em',
                 transform: function(d, i) {
                     var angle = currentAngle(d, i);
                     var rad = radius + axisConfig.labelOffset;