Draw major ticks along the X and Y axes.

Many thanks to Keith Packard for suggesting the math to
normalize and choose a step spacing.

diff --git a/acre.c b/acre.c
index 394b70035e9e5147516a7ec3ac3549c59edf7513..5949a96a1a38ccc37fe48397b0cb3ea3d755e046 100644 (file)
--- a/acre.c
+++ b/acre.c
@@ -151,7 +151,7 @@ acre_add_data (acre_t *acre, acre_data_t *data)
 #define ACRE_FONT_SIZE 12
 #define ACRE_TITLE_FONT_SIZE 32
 #define ACRE_PAD (ACRE_FONT_SIZE)
-#define ACRE_TICK_SIZE 4
+#define ACRE_TICK_SIZE 6
 
 static void
 _draw_title_and_labels (acre_t *acre)
@@ -273,16 +273,13 @@ _compute_axis_ranges (acre_t *acre)
     }
 }
 
+/* Setup a transformation in acre->cr such that data values plotted
+ * will appear where they should within the chart.
+ */
 static void
-_draw_data (acre_t *acre)
+_set_transform_to_data_space (acre_t *acre)
 {
     cairo_t *cr = acre->cr;
-    unsigned int d, i;
-    acre_data_t *data;
-
-    cairo_save (cr);
-
-    cairo_set_source_rgb (cr, 0, 0, 0);
 
     cairo_translate (cr,
                     acre->chart.x + 0.5,
@@ -291,6 +288,20 @@ _draw_data (acre_t *acre)
                 acre->chart.width / (acre->x_axis.max - acre->x_axis.min),
                 - acre->chart.height /(acre->y_axis.max - acre->y_axis.min));
     cairo_translate (cr, -acre->x_axis.min, -acre->y_axis.min);
+}
+
+static void
+_draw_data (acre_t *acre)
+{
+    cairo_t *cr = acre->cr;
+    unsigned int d, i;
+    acre_data_t *data;
+
+    cairo_save (cr);
+
+    cairo_set_source_rgb (cr, 0, 0, 0);
+
+    _set_transform_to_data_space (acre);
 
     for (d = 0; d < acre->num_data; d++) {
        data = acre->data[d];
@@ -312,18 +323,93 @@ _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)
 {
     cairo_t *cr = acre->cr;
+    double step, x, y;
+
+    cairo_save (cr);
+
+    cairo_set_source_rgb (cr, 0, 0, 0); /* black */
 
+    /* First the ticks within data space. */
     cairo_save (cr);
+    {
+       _set_transform_to_data_space (acre);
+
+       step = _step_for_range (acre->x_axis.max -acre->x_axis.min);
+       x = (floor (acre->x_axis.min / step) + 1) * step;
+       while (x <= acre->x_axis.max) {
+           cairo_move_to (cr, x, acre->y_axis.min);
+           cairo_save (cr);
+           {
+               cairo_identity_matrix (cr);
+               cairo_rel_line_to (cr, 0, 0.5);
+               cairo_rel_line_to (cr, 0, -ACRE_TICK_SIZE);
+               cairo_set_line_width (cr, 1.0);
+               cairo_stroke (cr);
+           }
+           cairo_restore (cr);
+           x += step;
+       }
 
+       step = _step_for_range (acre->y_axis.max -acre->y_axis.min);
+       y = (floor (acre->y_axis.min / step) + 1) * step;
+       while (y <= acre->y_axis.max) {
+           cairo_move_to (cr, acre->x_axis.min, y);
+           cairo_save (cr);
+           {
+               cairo_identity_matrix (cr);
+               cairo_rel_line_to (cr, -0.5, 0);
+               cairo_rel_line_to (cr, ACRE_TICK_SIZE, 0);
+               cairo_set_line_width (cr, 1.0);
+               cairo_stroke (cr);
+           }
+           cairo_restore (cr);
+           y += step;
+       }
+    }
+    cairo_restore (cr);
+
+    /* Then the frame drawn in pixel space. */
     cairo_rectangle (cr,
                     acre->chart.x - 0.5, acre->chart.y - 0.5,
                     acre->chart.width + 1.0, acre->chart.height + 1.0);
     cairo_set_line_width (cr, 1.0);
-    cairo_set_source_rgb (cr, 0, 0, 0);
     cairo_stroke (cr);
 
     cairo_restore (cr);