fix: heart rate plots with no operation

This fixes a bug that would crash the application when plotting
heart rate (i.e. `HKQuantityTypeIdentifierHeartRate`) data with no operation.

apple_health_parser/interfaces/plot_interface.py

@@ -67,16 +67,16 @@ def _get_plot_settings(self) -> PlotSettings:
         legend = None
         title = None
         title_yaxis = None
-
-        match self.flag:
-            case "HKQuantityTypeIdentifierHeartRate":
+        
+        # Special case for the heart rate flag ot include the motion context
+        if self.flag == "HKQuantityTypeIdentifierHeartRate" and self.oper is None:
                 x = "start_date"
                 y = "value"
                 color = "motion_context"
                 legend = "Motion Context"
-            case _:
-                x = "date"
-                y = "value"
+        else:
+            x = "date"
+            y = "value"
 
         match self.title:
             case True:
@@ -157,7 +157,7 @@ def plot(
                 figure.write_html(output_dir / f"{filename}.html")
             else:
                 figure.write_image(
-                    file=output_dir / f"{filename}.{format}", format=format, scale=1
+                    file=output_dir / f"{filename}.{format}", format=format, scale=2
                 )
 
         return figure