+ _show_layout (cr, x_axis_layout);
+
+ 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, int *minor_divisions)
+{
+ 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;
+ *minor_divisions = 4;
+ } else {
+ step = 2.5;
+ *minor_divisions = 5;
+ }
+ } else {
+ if (step < 7.071067811865475) {
+ step = 5.0;
+ *minor_divisions = 5;
+ } else {
+ step = 10.0;
+ *minor_divisions = 4;
+ }
+ }
+
+ /* 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 void
+_expand_range_for_width (double *axis_min, double *axis_max, int pixel_range)
+{
+ double range, new_range, step, step_minor, pixel_step;
+ int minor_divisions;
+
+ range = *axis_max - *axis_min;
+
+ step = _step_for_range (range, &minor_divisions);
+ step_minor = step / minor_divisions;
+
+ pixel_step = step_minor * (pixel_range / range);
+
+ /* We expand the range by the ratio of the pixel step to the floor
+ * of the pixel_step.
+ */
+ new_range = range * pixel_step / floor (pixel_step);
+
+ /* And spread the increase out on either side of the range. */
+ *axis_min -= (new_range - range) / 2.0;
+ *axis_max += (new_range - range) / 2.0;
+}
+
+/* Setup a transformation in acre->cr such that data values plotted
+ * will appear where they should within the chart.
+ */
+static void
+_set_transform_to_data_space (acre_t *acre)
+{
+ cairo_t *cr = acre->cr;
+
+ cairo_translate (cr,
+ acre->chart.x,
+ acre->chart.y + acre->chart.height);
+ cairo_scale (cr,
+ 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
+_compute_axis_ranges (acre_t *acre)
+{
+ unsigned int d, i;
+ acre_data_t *data;
+ double x_adjust, y_adjust;
+ cairo_t *cr = acre->cr;
+
+ acre->x_axis.min = acre->data[0]->points[0].x;
+ acre->x_axis.max = acre->data[0]->points[0].x;
+ acre->y_axis.min = acre->data[0]->points[0].y;
+ acre->y_axis.min = acre->data[0]->points[0].y;
+
+ /* 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++) {
+ if (data->points[i].x < acre->x_axis.min)
+ acre->x_axis.min = data->points[i].x;
+ if (data->points[i].x > acre->x_axis.max)
+ acre->x_axis.max = data->points[i].x;
+
+ if (data->points[i].y < acre->y_axis.min)
+ acre->y_axis.min = data->points[i].y;
+ if (data->points[i].y > acre->y_axis.max)
+ acre->y_axis.max = data->points[i].y;
+ }
+ }
+
+ /* Next, we want to ensure that the data never collides with the
+ * ticks. So we expand each axis on its minimum side as needed. */
+ cairo_save (cr);
+ {
+ double x, y;
+
+ _set_transform_to_data_space (acre);
+
+ x = ACRE_TICK_MAJOR_SIZE + 2.0;
+ y = ACRE_TICK_MAJOR_SIZE + 2.0;
+ cairo_device_to_user_distance (cr, &x, &y);
+
+ acre->x_axis.min -= x;
+ acre->y_axis.min += y;
+ }
+ cairo_restore (cr);
+
+ /* Then, increase the axis ranges just enough so that the step
+ * sizes for the ticks will be integers.
+ */
+ _expand_range_for_width (&acre->x_axis.min,
+ &acre->x_axis.max,
+ acre->chart.width);
+
+ _expand_range_for_width (&acre->y_axis.min,
+ &acre->y_axis.max,
+ acre->chart.height);
+
+ /* Finally, we also translate the axis ranges slightly so that the
+ * ticks land on half-integer device-pixel positions.
+ */
+ cairo_save (cr);
+ {
+ _set_transform_to_data_space (acre);
+
+ x_adjust = 0.0;
+ y_adjust = 0.0;
+ cairo_user_to_device (cr, &x_adjust, &y_adjust);
+ x_adjust = (round (x_adjust + 0.5) - 0.5) - x_adjust;
+ y_adjust = (round (y_adjust + 0.5) - 0.5) - y_adjust;
+ cairo_device_to_user_distance (cr, &x_adjust, &y_adjust);
+
+ acre->x_axis.min -= x_adjust;
+ acre->x_axis.max -= x_adjust;
+
+ acre->y_axis.min -= y_adjust;
+ acre->y_axis.max -= y_adjust;
+ }
+ cairo_restore (cr);
+}
+
+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];
+ cairo_new_path (cr);
+ for (i = 0; i < data->num_points; i++) {
+ cairo_line_to (cr,
+ data->points[i].x,
+ data->points[i].y);
+ }
+ cairo_save (cr);
+ {
+ cairo_identity_matrix (cr);
+ cairo_set_line_width (cr, 1.0);
+ cairo_stroke (cr);
+ }
+ cairo_restore (cr);
+ }
+
+ cairo_restore (cr);
+}
+
+typedef enum _ticks { ACRE_TICKS_X, ACRE_TICKS_Y } acre_ticks_t;
+
+static void
+_draw_ticks (acre_t *acre,
+ double axis_min, double axis_max,
+ acre_ticks_t ticks)
+{
+ cairo_t *cr = acre->cr;
+ double t, step, sub_step;
+ int minor_divisions;
+
+ cairo_save (cr);
+
+ _set_transform_to_data_space (acre);
+
+ step = _step_for_range (axis_max - axis_min, &minor_divisions);
+ sub_step = step / minor_divisions;
+
+ for (t = (floor (axis_min / sub_step) + 1) * sub_step;
+ t <= axis_max;
+ t += sub_step)
+ {
+ int tick_size;
+ if (fabs((t / step) - (int) (t / step)) < 0.5 * (sub_step / step))
+ tick_size = ACRE_TICK_MAJOR_SIZE;
+ else
+ tick_size = ACRE_TICK_MINOR_SIZE;
+
+ /* tick */
+ cairo_save (cr);
+ {
+ if (ticks == ACRE_TICKS_X)
+ cairo_move_to (cr, t, acre->y_axis.min);
+ else
+ cairo_move_to (cr, acre->x_axis.min, t);
+
+ cairo_identity_matrix (cr);
+
+ if (ticks == ACRE_TICKS_X) {
+ cairo_rel_line_to (cr, 0, 0.5);
+ cairo_rel_line_to (cr, 0, -tick_size - 0.5);
+ } else {
+ cairo_rel_line_to (cr, -0.5, 0);
+ cairo_rel_line_to (cr, tick_size + 0.5, 0);
+ }
+
+ cairo_set_line_width (cr, 1.0);
+ cairo_stroke (cr);
+ }
+ cairo_restore (cr);
+
+ /* label */
+ if (tick_size == ACRE_TICK_MAJOR_SIZE)
+ {
+ PangoLayout *layout;
+ int width, height;
+
+ cairo_save (cr);
+
+ layout = _create_layout_printf (acre, "%g", t);
+
+ if (ticks == ACRE_TICKS_X)
+ cairo_move_to (cr, t, acre->y_axis.min);
+ else
+ cairo_move_to (cr, acre->x_axis.min, t);
+
+ cairo_identity_matrix (cr);
+ pango_layout_get_pixel_size (layout, &width, &height);
+
+ if (ticks == ACRE_TICKS_X)
+ cairo_rel_move_to (cr, -width / 2, ACRE_X_TICK_VALUE_PAD);
+ else
+ cairo_rel_move_to (cr, -width - ACRE_Y_TICK_VALUE_PAD,
+ -height/2);
+
+ _show_layout (cr, layout);
+
+ cairo_restore (cr);
+ }
+ }
+
+ cairo_restore (cr);
+}
+
+static void
+_draw_frame_and_ticks (acre_t *acre)
+{
+ cairo_t *cr = acre->cr;
+
+ cairo_save (cr);
+
+ cairo_set_source_rgb (cr, 0, 0, 0); /* black */
+
+ /* ticks */
+ _draw_ticks (acre, acre->x_axis.min, acre->x_axis.max, ACRE_TICKS_X);
+ _draw_ticks (acre, acre->y_axis.min, acre->y_axis.max, ACRE_TICKS_Y);
+
+ /* frame */
+ 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_stroke (cr);
+
+ cairo_restore (cr);