BarChart: empty categoryColumn is treated as a real column, breaking px.bar

[kz930]

What happened?

BarChartOpDesc.categoryColumn declares a JSON default of "No Selection" but its Scala field initializer is the empty string:

common/workflow-operator/src/main/scala/org/apache/texera/amber/operator/visualization/barChart/BarChartOpDesc.scala

@JsonProperty(defaultValue = "No Selection", required = false)
...
var categoryColumn: EncodableString = ""      // Scala default is "", not "No Selection"

generatePythonCode decides whether a category column was chosen by comparing only against "No Selection":

var isCategoryColumn = "False"
if (categoryColumn != "No Selection")
  isCategoryColumn = "True"

When categoryColumn is empty — the Scala default, reached whenever the field is absent from the deserialized JSON (defaultValue is schema metadata and is not applied to the Scala var) — "" != "No Selection" is true, so isCategoryColumn = "True". The generated code then passes the empty column name to px.bar(color=...):

color=self.decode_python_template('') if True else None    # -> color=""  ->  px.bar(color="")

px.bar(color="") raises a KeyError/ValueError at runtime, so a bar chart with no category selected fails.

Expected: an unset/empty categoryColumn yields color=None (no category grouping).

How to reproduce?

A — user-facing: Add a Bar Chart operator, set only Value and Fields (leave the Category column unset), and run. The chart fails at runtime instead of rendering without a category.

B — confirmed against main's operator directly: construct BarChartOpDesc with value = "score", fields = "name", and categoryColumn left at its default (""), then call generatePythonCode(). The emitted call contains:

color=self.decode_python_template('') if True else None

i.e. isCategoryColumn is True for the empty column, so color is set to the empty-string column name → px.bar(color="").

Proposed fix: also guard against the empty value:

if (categoryColumn.nonEmpty && categoryColumn != "No Selection")
  isCategoryColumn = "True"

so an unset/empty categoryColumn yields color=None.

Version/Branch

1.3.0-incubating-SNAPSHOT (main)

Commit Hash (Optional)

No response

What browsers are you seeing the problem on?

No response

Relevant log output

[kz930]

/take