Lets say you want to draw the 9 * 9 grid on the screen - actually my cro has a 10 * 10 grid which I'll use, we have to ouput 'vectors' to draw the grid pretty much like you would with a pencil and paper. In this instance we can't control the on/off of the CRO trace so we can't lift the 'pencil' from the paper. If the CRO has 0-10V on the X and Y the voltage pairs would be -


0,0
10,0
10,1
0,1
0,2
10,2
10,3
0,3
0,4
10,4
10,5
0,5
0,6
10,6
10,7
0,7
0,8
10,8
10,9
0,9
0,10

this would draw all the horizontal lines. I won't continue for the vertical lines as I'll leave that for you. to make things easier, If you can control the CRO's 'z' input - this allows you to turn off the trace which is like 'lifting the pencil'. Anyway, your code would have a table of vectors like above and would output to the DAC these values for X and Y, pause say 1mS then output the next pair and so on until all the vectors have been drawn, then repeat.