Roll Observatory packages and add a roll script

packages/charted/lib/core/interpolators/interpolators.dart

 //
 // Copyright 2014 Google Inc. All rights reserved.
 //
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file or at
 // https://developers.google.com/open-source/licenses/bsd
 //
 
 part of charted.core.interpolators;
 
@@ skipping 372 matching lines @@
 Interpolator uninterpolateNumber(num a, num b) {
   b = 1 / (b - a);
   return (x) => (x - a) * b;
 }
 
 /// Reverse interpolator for a clamped number.
 Interpolator uninterpolateClamp(num a, num b) {
   b = 1 / (b - a);
   return (x) => math.max(0, math.min(1, (x - a) * b));
 }