Gridlines and axis labels make charts easier to understand. They give the lines a clear scale. Unless you tell it otherwise, dygraphs will choose a y-axis and set of gridlines which include all of your data.
If you have many series with different scales, this will compress the variation in all but the largest one. Standard ways to deal with this include secondary y-axes and log scales.
If neither of these is to your liking, you can manually rescale your series and undo that scaling for the hover values. This demo shows how to do it.
Hover over to see the original values. This is what the data looks like without any rescaling: