Topic: petit computer help

Hi all , This was posted else where in the forum but was in the wrong place and I lost the link to where it was supposed to be posted so I hope this place works .hope everyone is haveing a great weekend. My son brought his dsi to me the other day and showed me petit computer and was wondering what it was. I used to write stuff back in the day on an trs 80. i decided him and i should write some simple things to learn . I started out with a basic for-next loop and printed it to show him it counting . From there it progressed. When he gets an idea about a new way to show the numbers we add it to the menu and we write the chunk of code together for it . now we r stuck on the diagonal part. i got it to print the text starting out at 0,0 to 20,20. What i had looked something like this :

cls
for n=0to 100
x=n;y=n
locate x,y:?n
next n
wait 20
next n

it displays diagonaly to number 23 which is as tall as the screen is then prints in a column. how can i express that when x,y get to 20,20 to resest the count and count up again . also how would i go about making x,y count down from 20,20 to 0,0 then back. another thing i have been working on is displaying it in colors. i have the random color down but want to express the color like the number since color cant be greater than 15, i wanna reset color to 0 then count up again. i proved the qr codes to show the rest of the program and how it fits in . any help would be greatly appreciated.