If an ordinal domain is specified, any corresponding values that are not in the domain should be considered undefined.

[mbostock]

For example,

Plot.plot({
  x: {
    domain: "AEIOU"
  },
  y: {
    grid: true
  },
  marks: [
    Plot.barY(alphabet, {x: "letter", y: "frequency"}),
    Plot.ruleY([0])
  ]
})

Expected

Actual

[Fil]

If we want to generalize this issue, a scale that returns NaN for a datum should never have that datum pass the "filter" stage, and no element should be created in the selectAll().join.

But in the current model the scales are applied after the filter.

So the change might look like this (here I fixed only x, but the real fix would happen in all the Marks and for all the dimensions):

diff --git a/src/marks/bar.js b/src/marks/bar.js
index f38a589..09aa0e3 100644
--- a/src/marks/bar.js
+++ b/src/marks/bar.js
@@ -41,7 +41,9 @@ export class AbstractBar extends Mark {
   render(I, scales, channels, options) {
     const {color} = scales;
     const {z: Z, fill: F, stroke: S} = channels;
-    const index = filter(I, ...this._positions(channels), F, S);
+    let index = filter(I, ...this._positions(channels), F, S);
+    const x = Array.from(index, this._x(scales, channels, options));
+    index = filter(I, x);
     if (Z) index.sort((i, j) => ascending(Z[i], Z[j]));
     return create("svg:g")
         .call(applyIndirectStyles, this)
@@ -50,7 +52,7 @@ export class AbstractBar extends Mark {
           .data(index)
           .join("rect")
             .call(applyDirectStyles, this)
-            .attr("x", this._x(scales, channels, options))
+            .attr("x", i => x[i])
             .attr("width", this._width(scales, channels, options))
             .attr("y", this._y(scales, channels, options))
             .attr("height", this._height(scales, channels, options))

alternatively

diff --git a/src/marks/bar.js b/src/marks/bar.js
index f38a589..0822bf2 100644
--- a/src/marks/bar.js
+++ b/src/marks/bar.js
@@ -41,7 +41,8 @@ export class AbstractBar extends Mark {
   render(I, scales, channels, options) {
     const {color} = scales;
     const {z: Z, fill: F, stroke: S} = channels;
-    const index = filter(I, ...this._positions(channels), F, S);
+    const x = Array.from(I, this._x(scales, channels, options));
+    const index = filter(I, ...this._positions(channels), F, S, x);
     if (Z) index.sort((i, j) => ascending(Z[i], Z[j]));
     return create("svg:g")
         .call(applyIndirectStyles, this)
@@ -50,7 +51,7 @@ export class AbstractBar extends Mark {
           .data(index)
           .join("rect")
             .call(applyDirectStyles, this)
-            .attr("x", this._x(scales, channels, options))
+            .attr("x", i => x[i])
             .attr("width", this._width(scales, channels, options))
             .attr("y", this._y(scales, channels, options))
             .attr("height", this._height(scales, channels, options))

The second option makes a few more computations by applying _x to all indices, including those that would be rejected by the first filtering—but as a benefit the index i is true if the function to be called is (d,i) => something.

Systematically rejecting all elements for which whichever attribute would be computed as NaN (or null or undefined) probably makes sense?

[mbostock]

Yes, that sounds right: we should pass values through the scale to determine whether the scaled value is defined.