add Paul Heckbert's "Nice numbers for graph labels" algo

frontend/src/Lib.elm

@@ -0,0 +1,47 @@
+-- miscellaneous functions extracted from Main so
+-- we can more easily test them
+
+module Lib exposing(looseLabels)
+
+
+maybe default val =
+    case val of
+        Just v -> v
+        Nothing -> default
+
+-- https://github.com/cenfun/nice-ticks/blob/master/docs/Nice-Numbers-for-Graph-Labels.pdf
+
+log10 x = logBase 10 x
+expt b x = b^(toFloat x)
+
+niceNumber x round =
+    let exp = floor (log10 x)
+        f = x / (expt 10.0 exp)
+        nfRound = if f < 1.5
+                  then 1
+                  else if f < 3
+                       then 2
+                       else if f < 7
+                            then 5
+                            else 10
+        nf = if f <= 1
+             then 1
+             else if f <= 2
+                  then 2
+                  else if f <= 5
+                       then 5
+                       else 10
+    in
+        if round
+        then
+            nfRound * expt 10 exp
+        else
+            nf * expt 10 exp
+
+looseLabels ticks min max =
+    let
+        range = niceNumber (max-min) False
+        d = niceNumber (range/(ticks - 1)) True
+        graphmin = toFloat (floor  (min/d)) * d
+        graphmax = toFloat (ceiling (max/d)) * d
+    in (graphmin, graphmax, d)

frontend/tests/LibTest.elm

@@ -0,0 +1,31 @@
+module LibTest exposing (specs)
+
+import Lib exposing (..)
+import Test exposing (..)
+import Expect exposing (Expectation)
+
+specs: Test
+specs =
+    describe "looseLabels"
+        [ test "0-100" <|
+              \_ ->
+              let (u, v, _) = looseLabels 10 0.0 100.0
+              in Expect.equal (0, 100) (u, v)
+        , test "2-98" <|
+              \_ ->
+              let (u, v, _) = looseLabels 10  2 98
+              in Expect.equal (0, 100) (u, v)
+        , test "8-91" <|
+              \_ ->
+              let (u, v, _) = looseLabels 10  8 91
+              in Expect.equal (0, 100) (u, v)
+        , test "1-32" <|
+              \_ ->
+              let (u, v, _) = looseLabels 8  1 32
+              in Expect.equal (0, 40) (u, v)
+        , test "1-4" <|
+              \_ ->
+              let (u, v, _) = looseLabels 10  1 4
+              in Expect.equal (1, 4) (u, v)
+
+        ]