charts.lines: remove middle and its tests

lines/lines-tests.factor

@@ -220,10 +220,6 @@ IN: charts.lines.tests
     drawable-chunks [ { } like ] map
 ] unit-test
 
-{ 0 } [ -10 10 middle ] unit-test
-{ 15 } [ 10 20 middle ] unit-test
-{ -81/2 } [ -50 -31 middle ] unit-test
-
 {
     { { { 0 300 } { 1 200 } { 2 150 } { 3 100 } { 4 0 } } }
 } [

lines/lines.factor

@@ -199,10 +199,8 @@ ALIAS: y second
         monotonic-split-slice
     ] 2keep (drawable-chunks) ;
 
-: middle ( min max -- middle ) + 2 / ;
-
 : flip-y-axis ( chunks ymin,ymax -- chunks )
-    first2 middle 2 * '[ [ _ swap - ] assoc-map ] map ;
+    first2 + '[ [ _ swap - ] assoc-map ] map ;
 
 ! value' = (value - min) / (max - min) * width
 : scale ( width value max min -- value' ) neg [ + ] curry bi@ / * ;