Added ability to choose data values numerically
Refactored GridWindow::_key() by removing unnecessary return statements
Improved handling of current_value so that it can be 0
Removed extraneous "hello" from main()
Added a couple of other colour schemes
Made all function names into camel-case
Replaced array2d class with array-based representation
Switched to a more flexible mode
Solved zooming problem
Made polygons respond to click events
Added hexagonal grid cells