tutorial: point at vocabulary.md

tutorial/index.md

@@ -497,3 +497,6 @@ to make your programs easier to understand.
 Another thing to notice in this program is the commas. Commas are entirely
 optional in Mu, and it can be handy to drop them selectively to group
 arguments together.
+
+This is a good time to skim [Mu's vocabulary of functions for pixel graphics](https://github.com/akkartik/mu/blob/main/vocabulary.md#pixel-graphics).
+They're fun to play with.

vocabulary.md

@@ -227,6 +227,17 @@ Assertions for tests:
   at any screen location.
 - `check-background-color-in-screen-row-from`
 
+#### pixel graphics
+
+- `pixel`: draw a single point at (x, y) with a given color between 0 and 255.
+- `draw-line`: between two points (x1, y1) and (x2, y2)
+- `draw-horizontal-line`
+- `draw-vertical-line`
+- `draw-circle`
+- `draw-disc`: takes an inner and outer radius
+- `draw-monotonic-bezier`: draw curved lines with a single control point.
+  Doesn't support curves with "U-turns".
+
 #### events
 
 `read-key` reads a single key from the keyboard and returns it if it exists.
@@ -234,7 +245,11 @@ Returns 0 if no key has been pressed.
 
 `read-mouse-event` returns a recent change in x and y coordinate.
 
-Mu doesn't currently support interrupt-based mouse events.
+`timer-counter` returns a monotonically increasing counter with some
+fixed frequency. You can periodically poll it to check for intervals passing,
+but can't make assumptions about how much time has passed.
+
+Mu doesn't currently support interrupt-based events.
 
 #### persistent storage