Speed up waterfall drawing by scrolling more than one line at once

plot.c

@@ -2117,7 +2117,12 @@ static void update_waterfall(void){
   int i;
   int w_width = area_width < WIDTH ? area_width : WIDTH;
 //  START_PROFILE;
-  for (i = CHART_BOTTOM-1; i >=graph_bottom; i--) {		// Scroll down
+#define WATERFALL_MULTI (SPI_BUFFER_SIZE / WIDTH)
+  for (i = CHART_BOTTOM-WATERFALL_MULTI; i>=graph_bottom; i-=WATERFALL_MULTI) {		// Scroll down WATERFALL_MULTI lines at once
+    ili9341_read_memory(OFFSETX, i, w_width, WATERFALL_MULTI, spi_buffer);
+    ili9341_bulk(OFFSETX, i+1, w_width, WATERFALL_MULTI);
+  }
+  for (i = CHART_BOTTOM-((CHART_BOTTOM-graph_bottom)/WATERFALL_MULTI)*WATERFALL_MULTI-1; i>=graph_bottom; i--) {  // Scroll down remaining lines
     ili9341_read_memory(OFFSETX, i, w_width, 1, spi_buffer);
     ili9341_bulk(OFFSETX, i+1, w_width, 1);
   }