Basic drawing is now available in Day One iOS starting in version 3.4. You can use your finger or Apple Pencil (iPad only).
Tap to start a new blank entry, then tap the paper clip icon above the keyboard and choose Draw.
Or tap the + button at the bottom of the Timeline to choose Drawing.
In the drawing canvas, you can change the color or the writing utensil, change the opacity, and change the width. The white color serves as an eraser at this time.
Tap Done at the top right to save the drawing.
To edit a drawing, tap the drawing in the entry, then tap Edit.