To style your gridlines on your nvd3 plots (or Angular-nvd3) you can use simple css styles in the line object within your plot.

For example, to make the gridlines a dotted line, we could use the following styling:

.nvd3 .tick line {
    stroke-dasharray: 1,1;
}

Or to do a dashed line with 4 pixel long dashes separated by gaps of 2 pixels, we can do this:

.nvd3 .tick line {
    stroke-dasharray: 4,2;
}

We can of course use the same methods to apply lots of other stylings to our gridlines such as opacity (looks good on dark backgrounds), different colors, etc.