charts.lines: add calc-line-slope

lines/lines-tests.factor

@@ -1,8 +1,10 @@
 ! Copyright (C) 2017 Alexander Ilin.
 
-USING: tools.test charts.lines ;
+USING: tools.test charts.lines charts.lines.private ;
 IN: charts.lines.tests
 
+{ -2/3 } [ { 1 3 } { -2 5 } calc-line-slope ] unit-test
+
 { { } }
 [ { } { } clip-data ] unit-test
 

lines/lines.factor

@@ -2,7 +2,7 @@
 
 USING: accessors arrays binary-search charts
 combinators.short-circuit kernel locals math math.order
-math.statistics opengl opengl.gl sequences
+math.statistics math.vectors opengl opengl.gl sequences
 specialized-arrays.instances.alien.c-types.float ui.gadgets
 ui.render ;
 IN: charts.lines
@@ -61,6 +61,8 @@ TUPLE: line < gadget color data ;
 !        [ [ second ] dip second-in-bounds? ]
 !    } 2&& ;
 
+: calc-line-slope ( point1 point2 -- slope ) v- first2 swap / ;
+
 : clip-by-first ( min,max pairs -- pairs' )
     2dup first-in-bounds? [
         [ dup first  ] dip [ search-first? not ] keep