Draw axis if data crosses zero

[batteryviewer] / chart.c

diff --git a/chart.c b/chart.c
index 808a275289830ebf36c179c1063ab704055222ad..2d255e0cda8ab683fd1c1d441e39b2f5f8227236 100644 (file)
--- a/chart.c
+++ b/chart.c
@@ -21,9 +21,59 @@ struct BVChartPoint {
   float x, y;
 };
 
+struct ScreenPoint {
+  float x, y;
+};
+
 G_DEFINE_TYPE_WITH_CODE(BVChart, bv_chart, GTK_TYPE_DRAWING_AREA,
                         G_ADD_PRIVATE(BVChart))
 
+typedef void (*cairo_draw_func)(cairo_t *cr, double x, double y);
+
+static struct ScreenPoint to_screen(BVChartPrivate *priv,
+                                    GtkAllocation *allocation,
+                                    struct BVChartPoint *p) {
+  int margin = 3;
+  float xscale = (allocation->width - 2 * margin) / (priv->maxx - priv->minx);
+  float yscale = (allocation->height - 2 * margin) / (priv->miny - priv->maxy);
+  struct ScreenPoint screen_p = {
+      .x = xscale * (p->x - priv->minx) + margin,
+      .y = yscale * (p->y - priv->maxy) + margin,
+  };
+  return screen_p;
+}
+
+static void draw_data(BVChartPrivate *priv, cairo_t *cr,
+                      GtkAllocation *allocation) {
+  cairo_set_source_rgb(cr, 0.0, 0.0, 1.0);
+  float gap_threshold = 3.0;
+  float prevx = 0.0;
+  for (guint i = 0; i < priv->points->len; i++) {
+    struct BVChartPoint *p =
+        &g_array_index(priv->points, struct BVChartPoint, i);
+    gboolean is_gap = p->x - prevx > gap_threshold;
+    cairo_draw_func f = is_gap ? cairo_move_to : cairo_line_to;
+    struct ScreenPoint screen_p = to_screen(priv, allocation, p);
+    f(cr, screen_p.x, screen_p.y);
+    prevx = p->x;
+  }
+  cairo_stroke(cr);
+}
+
+static void draw_axis(BVChartPrivate *priv, cairo_t *cr,
+                      GtkAllocation *allocation) {
+  if (priv->miny < 0.0 && priv->maxy > 0.0) {
+    cairo_set_source_rgb(cr, 0.0, 0.0, 0.0);
+    struct BVChartPoint p = {.x = priv->minx, .y = 0.0};
+    struct ScreenPoint screen_p = to_screen(priv, allocation, &p);
+    cairo_move_to(cr, screen_p.x, screen_p.y);
+    p.x = priv->maxx;
+    screen_p = to_screen(priv, allocation, &p);
+    cairo_line_to(cr, screen_p.x, screen_p.y);
+    cairo_stroke(cr);
+  }
+}
+
 static gboolean bv_chart_draw(GtkWidget *widget, cairo_t *cr) {
   BVChart *chart = BV_CHART(widget);
   BVChartPrivate *priv = bv_chart_get_instance_private(chart);
@@ -37,20 +87,8 @@ static gboolean bv_chart_draw(GtkWidget *widget, cairo_t *cr) {
   GtkAllocation allocation;
   gtk_widget_get_allocation(widget, &allocation);
 
-  float xscale = allocation.width / (priv->maxx - priv->minx);
-  float yscale = allocation.height / (priv->miny - priv->maxy);
-  cairo_set_source_rgb(cr, 0.0, 0.0, 1.0);
-  struct BVChartPoint *start =
-      &g_array_index(priv->points, struct BVChartPoint, 0);
-  cairo_move_to(cr, xscale * (start->x - priv->minx),
-                yscale * (start->y - priv->maxy));
-  for (guint i = 1; i < priv->points->len; i++) {
-    struct BVChartPoint *p =
-        &g_array_index(priv->points, struct BVChartPoint, i);
-    cairo_line_to(cr, xscale * (p->x - priv->minx),
-                  yscale * (p->y - priv->maxy));
-  }
-  cairo_stroke(cr);
+  draw_axis(priv, cr, &allocation);
+  draw_data(priv, cr, &allocation);
 
   return TRUE;
 }