fix(vega): allow data item to miss optional field

[kristw]

This fix the following bug with data passed to pn.pane.Vega.

Example

dataset = [{"id": 1}, {"id":2, "parent": 1}, {"id": 3, "parent": 1}]

Expected behavior

The sample dataset that work in Vega examples works in Panel.

What happen

Panel app will crash with KeyError.

Why

The list of keys are extracted from the first item in the dataset only. (["id"] when it should be ["id", "parent"]),
This has to assume that all items in the dataset has the exact same fields, which is not always the case.
Therefore, when iterating through the 2nd and 3rd item, it cannot find data["parent"] to append the value to and throw KeyError.

Solution

Iterate through all items and compute the list of unique keys first.

[philippjfr]

Thanks for the fix!

[codecov]

Codecov Report

Merging #2853 (30fde4d) into master (c97b57e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2853   +/-   ##
=======================================
  Coverage   83.05%   83.05%           
=======================================
  Files         188      188           
  Lines       24454    24455    +1     
=======================================
+ Hits        20311    20312    +1     
  Misses       4143     4143           

Impacted Files	Coverage Δ
panel/pane/vega.py	82.78% <100.00%> (+0.14%)	⬆️

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