graphing calculator or scientific calculator

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I am in an advanced math class and will be enrolling in another one next year. I need a graphing calculator or a scientific one with advanced functions. I have seen a few apps in the app store for both, and am wondering if any of them are accessible with VoiceOver. I have some graphing calculator software installed on my computer, but it isn't very accessible. Does anyone know of an app or a stand alone graphing calculator that is accessible? Thanks so much, Holly