graph - filters

[daryldy]

add ability to filter graph results.

• time (i.e. only show readings from 4 am to 8 am)
• date (i.e. only show readings between two dates, only show readings for past week, etc)

each filter should have:

• user defined label
• different types of filters will have different sets of parameters (i.e. time ones will have start and end time, date ones will have starting and ending dates, etc)

user should be able to select which of their filters are to be used at any particular time