Deduplicate to_plot_f64 into a shared common helper

Co-authored-by: Copilot <copilot@github.com>

Author: kojix2

src/unicode_plot/common.cr

@@ -174,6 +174,13 @@ module UnicodePlot
     marker[0]
   end
 
+  private def to_plot_f64(values : Array(T)) : Array(Float64) forall T
+    {% unless T <= Number %}
+      {% raise "to_plot_f64 requires numeric array elements" %}
+    {% end %}
+    values.map(&.to_f64)
+  end
+
   def transform_name(tr : Symbol, basename : String = "") : String
     name = tr.to_s
     name == "identity" ? basename : "#{basename} [#{name}]"

src/unicode_plot/interface/lineplot.cr

@@ -1,11 +1,4 @@
 module UnicodePlot
-  private def to_plot_f64(values : Array(T)) : Array(Float64) forall T
-    {% unless T <= Number %}
-      {% raise "to_plot_f64 requires numeric array elements" %}
-    {% end %}
-    values.map(&.to_f64)
-  end
-
   def lineplot(
     x : Array(Float64),
     y : Array(Float64),

src/unicode_plot/interface/scatterplot.cr

@@ -1,11 +1,4 @@
 module UnicodePlot
-  private def to_plot_f64(values : Array(T)) : Array(Float64) forall T
-    {% unless T <= Number %}
-      {% raise "to_plot_f64 requires numeric array elements" %}
-    {% end %}
-    values.map(&.to_f64)
-  end
-
   def scatterplot(
     x : Array(Float64),
     y : Array(Float64),