clear button, property selector

• add property selector #4
• add button to clear graph #6
• label displays property used if name not provided #8

bug fixes, cleanup

• fix #3 resize chart width when width of y-axis changes
• fix #2 hide chart on deactivate as an option
• fix #5 improve configuration layout
• code cleanup, remove console output

Initial release

Display a simple time series chart in the flow editor.