OFFSET was renamed to OFFSET_Y in ea5a30d9 ("add pause-mode"), while
the call to `draw_leds()` in bc7e3b92 ("Add led activity to ECG app")
still uses the old name. This causes an exception and the ECG app fails
to proceed past the start screen.

with longer 1s press on the TOP_RIGHT buttont you can pause the screen
update. You can then scroll through the increases 4s buffer with
BUTTON_LEFT and BOTTOM_RIGHT. Another press on TOP_RIGHT brings you back
into the normal mode.

The logging operation doesn't get interrupted when pausing the screen.