How much emphasis should be put on programming ability in a letter of recommendation for CS graduate studies? I have a little computer science background.  In one of my statistic courses we did significant data analysis with SAS and R coding.  I'm getting this professor to write a LOR for me.  He wants me to draft the LOR for him.

How should I write this letter so I don't come across as someone who confuses computer science with coding?

In other words, I want to convey how this course makes me a good candidate for a computer science graduate program.
Let us put aside the question of you vs. your letter writer, because the same question could apply to your statement of purpose.

The notion of "programming ability" per se is a bit of a red herring.  The problem is: how do you actually measure what a "good" programmer is?  The only meaningful answer that I know of is to illustrate what somebody has accomplished that required programming.  Any other assertion of programming ability, I regard as a red flag.  In fact, one of my litmus tests for whether somebody is a good programmer is to ask what languages they know: if they can readily and simply answer the question, they probably aren't a good programmer (the logic behind this: getting significant things done with software tends to, over time, involve being forced to deal with lots of random types of code in order to get things done).

Thus, I would recommend you instead focus on emphasizing what you have accomplished using programming.  This way, you will show not just "programming ability" but also personal organization, ability to accomplish difficult and complex tasks, ability to think in a computer science manner about problems, and other such qualities that actually matter a lot more for demonstrating your potential in a program.  If you did a cool and complex project in the class, that is what you should be talking about, with SAS and R being mentioned in passing as the tools you used.  If all you did was problem sets, well, I would have a hard time seeing how this could be a strong LOR in the first place.