Recently bought TouchDraw per suggestion and find amazing easy to use. Just entered a flowchart from an old project and learned the basics in the process. I revisited LucidChart, which was my earlier suggestion. I find it unusable on my iPad1. I previously used it on a desktop with Chrome browser. So my earlier comments were based on desktop use and not with real iPad experience.
There is another flow chart app I looked at, inFlowchart looks promising, but there is no import/export with Visio. TouchDraw had both Visio and DropBox support. Even the online LucidChart solution only does Visio import, but not export. I can't vouch for TouchDraw Visio support because I have an old Visio version at work.
That said, I can certain do some mid size charts with TouchDraw.