feat: define polars_to_vega

Little work to do, but we do need to remove NaNs as vega does not
support them.

lib/syskit/log/dsl.rb

@@ -648,6 +648,11 @@ def roby_vega_task_timeline_data(*tasks)
                 end
             end
 
+            # Convert a Daru frame into a vega data array
+            def polars_to_vega(frame)
+                frame.fill_nan(nil).to_a
+            end
+
             # Convert a Daru frame into a vega data array
             def daru_to_vega(frame, every: 1)
                 data = []

test/dsl_test.rb

@@ -502,6 +502,20 @@ def assert_polars_frame_near(expected, actual)
                 end
             end
 
+            describe "polars_to_vega" do
+                before do
+                    @context = make_context
+                end
+
+                it "converts NaN into nil" do
+                    frame = ::Polars::DataFrame.new(
+                        { "a" => [Float::NAN, 0.1] }, schema: nil
+                    )
+                    vega = @context.polars_to_vega(frame)
+                    assert_equal [{ "a" => nil }, { "a" => 0.1 }], vega
+                end
+            end
+
             describe "daru_to_vega" do
                 before do
                     @context = make_context