How to steer a research fellowship back to the main initial focus rather than the supervisor's programming needs? Currently, I work as a part time (0.2 FTE) fixed-term (1 year) Research Fellow in the field of public health - this is not my primary area of research or expertise, but the project looked interesting and had some mutual research interests - primarily the data analytics aspects.

However, as time has gone by (half way), all I have done is programming the primary tool for data collection (Android based), and now the supervisor is started to strongly go on about loosely-relevant web design for another one of her projects.

I have declined to do the web design, as I am not proficient in it and simply not interested - also reminding her that web design was not part of the original plan, nor do I possess the qualifications or experience.

She has mentioned that she has collected data for the main project and in her words 'to save me time', has had a grad student analyse it.  When I politely reminded her that this data analytics was what I was employed to do and are qualified in, she ignores the email and goes on about the web design (and about superficial improvements to the Android program).

How can I steer a research fellowship back to the main initial focus (data analytics) rather than the supervisor's programming needs?
This fits in the standard mold, I think, of "How do I assert myself with my advisor?"

You wrote one assertive email.  At this point you can choose between a more strongly worded (but still polite, of course) email, and asserting yourself in person (preferably in a special appointment).

Personally I would choose the latter in most cases.

Asserting oneself works best in a stair-step escalation.  Each step up must be discernably stronger than the previous, but it's best to avoid going from zero to 60, to save face.

Keep the conversation as neutral in tone and as brief as possible.