DIAGRAM: fix drawing for negative coordinates

Applying the filter function too late seemed to cause
glitches in convexHStepsLs for negative coordinates.
We should only pass the filtered list of points to
the graham scan.

Graphics/Diagram/Plotter.hs

@@ -48,11 +48,11 @@ convexHP :: Diag
 convexHP = Diag chp
   where
     chp p vt =
-      position (zip (filter (inRange (dX p) (dY p)) vtch)
+      position (zip vtch
         (repeat dot))
       where
         dot = (circle $ t p :: Diagram Cairo R2) # fc red # lc red
-        vtch = grahamCH vt
+        vtch = grahamCH $ filter (inRange (dX p) (dY p)) vt
 
 
 -- |Show coordinates as text above the convex hull points.
@@ -94,16 +94,15 @@ convexHStepsLs :: Colour Double
                -> [PT]
                -> [Diagram Cairo R2]
 convexHStepsLs col f p xs =
-  fmap mkChDiag (f xs)
+  fmap mkChDiag (f xs')
   where
+    xs' = filter (inRange (dX p) (dY p)) xs
     mkChDiag vt =
       (strokeTrail         .
           fromVertices     $
-          vtf)            #
-        moveTo (head vtf) #
+          vt)            #
+        moveTo (head vt) #
         lc col
-      where
-        vtf = filter (inRange (dX p) (dY p)) vt
 
 
 -- |Create list of diagrama which describe the lines along the lower