Some Plot properties can't be dynamic?

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1

Trying to adjust the position of text based on where it would be placed… is it not possible to do this?

      Plot.text(data, Plot.selectMaxY({
        x: "time", y: "value",
        text: d => `Observed Max: ${d.value}`,
        dy: d => d.time < x_axis_midpoint ? -2 :  2,
        dx: d => d.time < x_axis_midpoint ?  2 : -2,
        textAnchor: d => d.time < x_axis_midpoint ? "start" : "end",
        frameAnchor: "bottom",
        fill: "red",
        fontSize: label_size,
      })),

When I try this, I literally get d => d.time < x_axis_midpoint ? "start" : "end", as the value of the text-anchor attribute.