Free

• Core Plot - https://code.google.com/p/core-plot/, http://www.raywenderlich.com/13269/how-to-draw-graphs-with-core-plot-part-1
• iOSPlot - https://github.com/honcheng/iOSPlot
• Motion Plot - JSON wrapper around Core Plot - https://github.com/toamitkumar/motion-plot

Paid

• ShinobiCharts - http://www.shinobicontrols.com/ios/shinobicharts/product-tour/
• iOS:Chart by Three D Graphics - http://www.threedgraphics.com/tdg/products/tools/ioschart/
• KeepEdge - http://www.keepedge.com/products/iphone_charting/
• NChart3D - http://nchart3d.com/

Javascript and a UIWebView

• http://www.highcharts.com/demo/pie-basic
• http://www.chartjs.org/
• http://www.fusioncharts.com/javascript-charting-comparison/, http://blog.fusioncharts.com/2012/02/create-charts-for-iphone-and-ipad-apps-using-fusioncharts-xt/
• http://g.raphaeljs.com/
• http://www.flotcharts.org/

Articles

• http://flaviusim.com/blog/2012/05/15/ios-chart-and-plotting-library-comparison/
• http://www.scottlogic.com/blog/2012/05/21/comparison-of-the-available-ios-charting-libraries.html
• https://blog.serverdensity.com/how-to-build-stunning-custom-ios-graphs-on-iphone-and-ipad/