interactive-graph: add padding above label

Summary:
Prevents "click to add points" and the y-axis label from
overlapping each other.

Test Plan:
load an [example item](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22Hector%20wanted%20to%20know%20if%20the%20amount%20of%20coffee%20he%20drinks%20in%20the%20morning%20affects%20the%20time%20it%20takes%20him%20to%20get%20to%20work.%20%20%5Cn%20%20%5CnEvery%20day%2C%20he%20recorded%20the%20amount%20of%20coffee%20he%20drank%20(in%20milliliters)%20and%20the%20duration%20of%20the%20drive%20(in%20minutes).%20%20%5Cn%20%20%5Cn**Plot%20the%20data%20in%20a%20scatter%20plot.**%20%20%5Cn%20%20%5Cn%7C%7C%7C%7C%7C%7C%5Cn%3A--%20%7C%20--%3A%20%7C%20--%3A%20%7C%20--%3A%20%7C%20--%3A%20%7C%20--%3A%20%7C%20--%3A%20%7C%20--%3A%20%7C%20--%3A%20%7C%20--%3A%20%7C%20--%3A%5Cn**Coffee**%20%7C%20%24100%24%20%7C%20%24200%24%20%7C%20%24150%24%20%7C%20%24125%24%20%7C%20%24225%24%5Cn**Duration**%20%7C%20%2418%24%20%7C%20%2414%24%20%7C%20%2415%24%20%7C%20%2420%24%20%7C%20%2412%24%20%5Cn%5Cn%20%5Cn%5B%5B%E2%98%83%20interactive-graph%201%5D%5D%20%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22interactive-graph%201%22%3A%7B%22type%22%3A%22interactive-graph%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22step%22%3A%5B50%2C2%5D%2C%22backgroundImage%22%3A%7B%22url%22%3A%22%22%2C%22scale%22%3A1%2C%22bottom%22%3A0%2C%22left%22%3A0%2C%22width%22%3A0%2C%22height%22%3A0%7D%2C%22markings%22%3A%22graph%22%2C%22labels%22%3A%5B%22%5C%5Ctext%7BCoffee%7D%22%2C%22%5C%5Ctext%7BDuration%7D%22%5D%2C%22showProtractor%22%3Afalse%2C%22showRuler%22%3Afalse%2C%22rulerLabel%22%3A%22%22%2C%22rulerTicks%22%3A10%2C%22range%22%3A%5B%5B-25%2C300%5D%2C%5B-2%2C21%5D%5D%2C%22gridStep%22%3A%5B50%2C1%5D%2C%22snapStep%22%3A%5B25%2C0.5%5D%2C%22graph%22%3A%7B%22type%22%3A%22point%22%2C%22numPoints%22%3A%22unlimited%22%7D%2C%22correct%22%3A%7B%22type%22%3A%22point%22%2C%22numPoints%22%3A%22unlimited%22%2C%22coords%22%3A%5B%5B100%2C18%5D%2C%5B200%2C14%5D%2C%5B150%2C15%5D%2C%5B125%2C20%5D%2C%5B225%2C12%5D%5D%7D%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22Drag%20the%20points%20to%20graph%20the%20answer.%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22hints%22%3A%5B%7B%22content%22%3A%22Let's%20plot%20each%20point%2C%20with%20its%20%24x%24-coordinate%20coming%20from%20the%20amount%20of%20*coffee*%20and%20the%20%24y%24-coordinate%20coming%20from%20the%20corresponding%20*duration*.%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22The%20final%20graph%20looks%20like%20this%3A%20%20%5Cn%20%20%5Cn!%5B%5D(https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F03f1b2090a331efd4cfa315c428fc5fe27d2bdc1.png)%22%2C%22images%22%3A%7B%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F03f1b2090a331efd4cfa315c428fc5fe27d2bdc1.png%22%3A%7B%22width%22%3A425%2C%22height%22%3A425%7D%7D%2C%22widgets%22%3A%7B%7D%7D%5D%7D)

Auditors: charlie