tick scaling: Added comments, and adjust loop ends

Update the mults arrays to cover the range from 1..base^2, and start the loop at a slightly lower value for safety. This should help ensure equivalent behavior to the pre-patch version.

diff --git a/dygraph-tickers.js b/dygraph-tickers.js
index 457d23c..80eb291 100644 (file)
--- a/dygraph-tickers.js
+++ b/dygraph-tickers.js
@@ -157,17 +157,31 @@ Dygraph.numericTicks = function(a, b, pixels, opts, dygraph, vals) {
       var kmg2 = opts("labelsKMG2");
       var mults, base;
       if (kmg2) {
-        mults = [1, 2, 4, 8, 16];
+        mults = [1, 2, 4, 8, 16, 32, 64, 128, 256];
         base = 16;
       } else {
-        mults = [1, 2, 5, 10];
+        mults = [1, 2, 5, 10, 20, 50, 100];
         base = 10;
       }
 
-      var wanted_ticks = pixels / pixels_per_tick;
-      var units_per_tick = Math.abs(b - a) / wanted_ticks;
+      // Get the maximum number of permitted ticks based on the
+      // graph's pixel size and pixels_per_tick setting.
+      var max_ticks = Math.ceil(pixels / pixels_per_tick);
+
+      // Now calculate the data unit equivalent of this tick spacing.
+      // Use abs() since graphs may have a reversed Y axis.
+      var units_per_tick = Math.abs(b - a) / max_ticks;
+
+      // Based on this, get a starting scale which is the largest
+      // integer power of the chosen base (10 or 16) that still remains
+      // below the requested pixels_per_tick spacing.
       var base_power = Math.floor(Math.log(units_per_tick) / Math.log(base));
       var base_scale = Math.pow(base, base_power);
+
+      // Now try multiples of the starting scale until we find one
+      // that results in tick marks spaced sufficiently far apart.
+      // The "mults" array should cover the range 1 .. base^2 to
+      // adjust for rounding and edge effects.
       var scale, low_val, high_val, nTicks, spacing;
       for (j = 0; j < mults.length; j++) {
         scale = base_scale * mults[j];