Implement separate class for rendering Skew-T legends

lib/xmet/skew_t.py

@@ -13,7 +13,7 @@ PRESSURE_LOG_MAX   = math.log(PRESSURE_MAX)
 PRESSURE_LOG_MIN   = math.log(PRESSURE_MIN)
 PRESSURE_LOG_RANGE = PRESSURE_LOG_MAX - PRESSURE_LOG_MIN
 
-TEMP_MAX        =  50
+TEMP_MAX        =  50 # degrees C
 TEMP_MIN        = -50
 TEMP_RANGE      = TEMP_MAX - TEMP_MIN
 TEMP_CENTER     =   0 # degrees C
@@ -131,3 +131,61 @@ class SkewTGraph():
         self.draw_samples(cr, x, y, samples, lambda s: s.dewpoint)
 
         cr.reset_clip()
+
+class SkewTLegend():
+    @staticmethod
+    def draw_isotherm_legends_for_graph(cr: cairo.Context,
+                                        skew_t: SkewTGraph,
+                                        x: float,
+                                        y: float):
+        temp = TEMP_MAX
+
+        while True:
+            text    = "%d°C" % (temp)
+            extents = cr.text_extents(text)
+
+            x_rel, y_rel = skew_t.sample_to_screen(temp, PRESSURE_MAX)
+
+            if x_rel < 0:
+                y_rel = skew_t.height + x_rel
+                x_rel = 0
+
+                if y_rel <= 0:
+                    break
+
+                cr.move_to(x + x_rel - extents.width - 8,
+                           y + y_rel + extents.height / 2)
+            else:
+                cr.move_to(x + x_rel - extents.width / 2,
+                           y + y_rel + extents.height + 8)
+
+            cr.show_text(text)
+            cr.stroke()
+
+            temp -= 2 * TEMP_STEP
+
+    @staticmethod
+    def draw_isobar_legends_for_graph(cr: cairo.Context,
+                                      skew_t: SkewTGraph,
+                                      x: float,
+                                      y: float):
+        for pressure in range(PRESSURE_MAX, PRESSURE_MIN-1, -2*PRESSURE_STEP):
+            x_rel = skew_t.width
+            y_rel = skew_t.height - skew_t.pressure_to_y(pressure)
+
+            text    = "%dmb" % (pressure)
+            extents = cr.text_extents(text)
+
+            cr.move_to(x + x_rel + 8,
+                       y + y_rel + extents.height / 2)
+
+            cr.show_text(text)
+            cr.stroke()
+
+    @staticmethod
+    def draw_for_graph(cr: cairo.Context,
+                       skew_t: SkewTGraph,
+                       x: float,
+                       y: float):
+        SkewTLegend.draw_isotherm_legends_for_graph(cr, skew_t, x, y)
+        SkewTLegend.draw_isobar_legends_for_graph(cr, skew_t, x, y)