Cleverly adjust axis ranges for integer tick spacing.

This will help avoid one of my major pet peeves with chartting
packages, which is non-pixel-aligned ticks. This is part of
the problem solved, (integer spacing between ticks), but we
still need integer alignment.

diff --git a/acre.c b/acre.c
index 0dcf8044ab8a009e2bb2076656998dbd73aed51a..6a4728ba99313b8798fdd793000e4c03e601ccc4 100644 (file)
--- a/acre.c
+++ b/acre.c
@@ -251,12 +251,68 @@ _draw_title_and_labels (acre_t *acre)
     cairo_restore (cr);
 }
 
+/* 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);
+}
+
 static void
 _compute_axis_ranges (acre_t *acre)
 {
     unsigned int d, i;
     acre_data_t *data;
+    double x_range, new_x_range;
+    double y_range, new_y_range;
 
+    /* First, simply find the extrema of the data. */
     for (d = 0; d < acre->num_data; d++) {
        data = acre->data[d];
        for (i = 0; i < data->num_points; i++) {
@@ -271,6 +327,22 @@ _compute_axis_ranges (acre_t *acre)
                acre->y_axis.max = data->points[i].y;
        }
     }
+
+    /* Next, increase the axis ranges just enough so that the step
+     * sizes for the ticks will be integers.
+     */
+    x_range = acre->x_axis.max - acre->x_axis.min;
+    new_x_range = _expand_range (x_range, acre->chart.width);
+
+    y_range = acre->y_axis.max - acre->y_axis.min;
+    new_y_range = _expand_range (y_range, acre->chart.height);
+
+    /* And spread the increase out on either side of the range. */
+    acre->x_axis.min -= (new_x_range - x_range) / 2.0;
+    acre->x_axis.max += (new_x_range - x_range) / 2.0;
+
+    acre->y_axis.min -= (new_y_range - y_range) / 2.0;
+    acre->y_axis.max += (new_y_range - y_range) / 2.0;
 }
 
 /* Setup a transformation in acre->cr such that data values plotted
@@ -323,39 +395,6 @@ _draw_data (acre_t *acre)
     cairo_restore (cr);
 }
 
-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;
-}
-
 static void
 _draw_frame_and_ticks (acre_t *acre)
 {