[RFR] Put labels over drops

[jpetitcolas]

In some case (for instance in #51), we may need to display labels inside drop lines instead of outside them. It is achievable using a negative label padding, such as:

const chart = eventDrops({
    label: {
        padding: -200,
    },
};

Yet, before this PR, drops were above the labels, causing some display issues:

With these changes, the text would be above the drops:

Closes #51.