Add Legend to Leaflet Map

The legend does not depend on the existence of any particular layer. The part that you are missing here is a getColor function. Copying the one from the tutorial in question:

function getColor(d) {
    return d > 1000 ? '#800026' :
           d > 500  ? '#BD0026' :
           d > 200  ? '#E31A1C' :
           d > 100  ? '#FC4E2A' :
           d > 50   ? '#FD8D3C' :
           d > 20   ? '#FEB24C' :
           d > 10   ? '#FED976' :
                      '#FFEDA0';
}

You see that the breaks are the same as the grades specified in the legend control:

grades = [0, 10, 20, 50, 100, 200, 500, 1000]

If you adjust these breakpoints to your needs and set the colors accordingly, you will have the legend you want.