If you're currently in the market for a relatively inexpensive iPhone and iPad app that will allow you to perform various geometric functions then you'll definitely want to take a look at TouchPlot as it could be just what you've been looking for.
What we have here is an expertly developed app in the Education category of the App Store that will allow you to perform complex geometric and algebraic calculations with custom keyboards thanks to its vast collection of functions.
This app offers support for drawn equations, contour plots, gradient fields, 3D plotting, inequation solutions, and so on, plus logarithmic scales are also provided and can be adjusted on both horizontal and vertical axes.
TouchPlot allows you to define your own constants with sliders to fully customize the way your curves respond to parameter changes; all of this in addition to parametric curves which are displayed in Cartesian and polar coordinates.
I honestly don't have much of a technical understanding of how to use the tools in this app but I do know the value of an accurate utility when it comes to performing geometric equations so I think you'll be quite pleased with what you find here.