Matplotlib!

GeoGebra is free and multi-platform dynamic mathematics software for learning and teaching. It has received several educational software awards in Europe and the USA.

Gnuplot is a portable command-line driven interactive data and function plotting utility. It was originally intended as to allow scientists and students to visualize mathematical functions and data. It does this job pretty well, but has grown to support many non-interactive uses, including web scripting and integration as a plotting engine for third-party applications like Octave. Gnuplot supports many types of plots in either 2D and 3D. It can draw using lines, points, boxes, contours, vector fields, surfaces, and various associated text. It also supports various specialized plot types. »

Plotly is the easiest way to graph and share data online. Open-source libraries for JavaScript (comparison with HighCharts at https://plot.ly/highcharts-alternative/ ), Python, R, and MATLAB.

Microsoft Mathematics is a desktop graphing calculator that can help you visualize and see mathematical concepts as you’ve never seen them before. Microsoft Mathematics includes ink handwriting support to recognize hand-written problems.

Bokeh is a Python interactive visualization library that targets modern web browsers for presentation. Its goal is to provide elegant, concise construction of novel graphics in the style of D3.js, but also deliver this capability with high-performance interactivity over very large or streaming datasets. Bokeh can help anyone who would like to quickly and easily create interactive plots, dashboards, and data applications.

Giac/Xcas is a free computer algebra system for Windows, Mac OS X and Linux/Unix (license GPL3, for commercial dual-license contact us).

Dash Reports is a versatile and light weight reporting solution. It allows users and administrators to create a large range of reports driven by SQL databases.

GraphCalc is an all-in-one solution to everything from everyday arithmetic to statistical analysis, from betas to Booleans, from cubes to calculus, from decimals to derivatives. GraphCalc combines all the features of a professional mathematics package with the simplicity of an easy to learn windows interface. It provides user-friendly help and tutorials to guide you through the easy and fun process of mastering GraphCalc.

DataGraph is a simple and powerful graphing application for Mac OS X. It is a great companion for Excel, Numbers or any of the big statistical packages.

SciDAVis is a free application for Scientific Data Analysis and Visualization. It's an application aimed at data analysis andpublication-quality plotting. It combines a shallow learning curve andan intuitive, easy-to-use graphical user interface with powerfulfeatures such as scriptability and extensibility. Quite extensive manual can be found here: http://scidavis.sourceforge.net/help/manual/index.html

ggvis is a data visualization package for R which lets you:

OmniGraphSketcher helps you make elegant and precise graphs in seconds, whether you have specific data to visualize or you just have a concept to explain. Specifically designed for reports, presentations, and problem sets where you need to produce sharp-looking graphs on the fly, OmniGraphSketcher combines the data plotting power of charting applications with the ease of a basic drawing program.