Use $.isNumeric instead of typeof == number

`typeof n === 'number'` yields `true` even for `Infinity` and `NaN`.
Hence use [isNumeric](http://api.jquery.com/jQuery.isNumeric/) to determine if operand is a number

_Note_: This PR contains these changes only for `jquery.flot.js`. If merged, I will make changes in the other files as well.