Update app.py

app.py

@@ -145,7 +145,8 @@ if run_sim:
         x_vals = states[:frame+1, 1] / 1000  # convert m to km
         y_vals = states[:frame+1, 2] / 1000  # convert m to km
         trajectory_line.set_data(x_vals, y_vals)
-        payload_marker.set_data(states[frame, 1] / 1000, states[frame, 2] / 1000)
+        # Wrap the current marker coordinates in a list so they are interpreted as sequences:
+        payload_marker.set_data([states[frame, 1] / 1000], [states[frame, 2] / 1000])
         time_text.set_text(f"Time: {t_val:.1f} s")
         return trajectory_line, payload_marker, time_text