+/* For a given axis range, compute a step size (in data space) to
+ * generate a suitable number of ticks (5 or so). */
+static double
+_step_for_range (double range)
+{
+ double step, scale_factor;
+
+ /* We want roughly 5 major ticks for the chart. */
+ step = range / 5;
+
+ /* Normalize the step so we can easily snap it to a desirable
+ * value. */
+ scale_factor = pow (10.0, floor (log10 (step)));
+ step /= scale_factor;
+
+ /* We want increments of 1, 2.5, 5, or 10 (times some power of
+ * 10). The threshold values between these are computed
+ * logarithmically. */
+ if (step < 3.535533905932738) {
+ if (step < 1.58113883008419)
+ step = 1.0;
+ else
+ step = 2.5;
+ } else {
+ if (step < 7.071067811865475)
+ step = 5.0;
+ else
+ step = 10.0;
+ }
+
+ /* Un-normalize and we now have the data value that we want to
+ * step at. */
+ return step * scale_factor;
+}
+
+/* Given an axis range, we can compute a desired data-space step
+ * amount for the major ticks (see _step_for_range). To get
+ * nice-looking pixel-snapped ticks we want to expand the range
+ * slightly. */
+static double
+_expand_range (double data_range, int pixel_size)
+{
+ double step, pixel_step;
+
+ step = _step_for_range (data_range);
+ pixel_step = step * pixel_size / data_range;
+
+ /* We expand the range by the ratio of the pixel step to the floor
+ * of the pixel_step.
+ */
+ return data_range * pixel_step / floor (pixel_step);
+}
+