Dygraphs can display numbers in either scientific mode (fixed number of significant figures) or fixed-point mode (fixed number of digits after the decimal point). It is in fixed-point mode by default.
To switch to scientific mode, set the sigFigs option to the number of significant figures in your data.
In fixed-point mode, you can control the number of digits after the decimal using the digitsAfterDecimal option. For particularly large numbers, this format can get unwieldy (i.e. '100000000' for 100M is a bit lengthy). Once the numbers get to a certain length, dygraphs will switch over to scientific notation. This is controlled by the maxNumberWidth option.