explicit error message for selectMaxX when channel x is missing

```js
Plot.text(
      leadtimes,
      Plot.selectMaxX({
        y: quantile,
        dy: -5,
        text: d => `85% (${quantile.toFixed(1)})`,
        textAnchor: "end",
        fill: "red",
      })
```

errors in `TypeError: undefined is not an object (evaluating 'n[t]')`

(via Martien)