$attachImage is the terminal operation for every canvas block. It triggers the deferred block flush: all queued drawing operations are executed in order, the result is rendered to a PNG, and the image is attached to the response. The name parameter becomes the filename (without extension) and is available at runtime via ((temp._canvasAttachment_name)). You can produce multiple images in a single command by creating multiple canvas blocks, each terminated by its own $attachImage call. Without $attachImage, the canvas block is never flushed and no image is produced.