I know there is astro charts but I don't trust it. There's no login or anything to secure your information. Anyone else know of any??
Could you download a chart, write your own interpretation, and save it somewhere?
How important to you is it that this be a digital diary? Personally, I find pen and paper to work, and I've used it in my own learning: search out what a few online sources say, copy them down, then follow with my own observations/interpretation/experience.
If it's just for you, then it can be any format, can't it? If you don't want to use pen and paper, you can do it in Google Docs or any word processing program.
If you want to share your interpretations with others, that might require digital tools, but even then, you can take a chart created on an app and put it somewhere else. I do that on my blog, for the Ask the Astrologer feature (see my signature): create the chart, download it, write my interpretation, and upload both to the blog.