Update charted to 0.4.8 and roll

packages/charted/lib/locale/format/number_format.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.locale.format;
 
 /**
@@ skipping 123 matching lines @@
       } else if (type == 'e' || type == 'f') {
         precision = math.max(0, math.min(20, precision));
       }
     }
 
     NumberFormatFunction formatFunction = _getFormatFunction(type);
 
     var zcomma = (zfill != null) && comma;
 
     return (value) {
+      if (value == null) return '-';
       var fullSuffix = suffix;
 
       // Return the empty string for floats formatted as ints.
       if (integer && (value % 1) > 0) return '';
 
       // Convert negative to positive, and record the sign prefix.
       var negative;
       if (value < 0 || value == 0 && 1 / value < 0) {
         value = -value;
         negative = '-';
@@ skipping 85 matching lines @@
       case 'e':
         return (num x, [int p = 0]) => x.toStringAsExponential(p);
       case 'f':
         return (num x, [int p = 0]) => x.toStringAsFixed(p);
       case 'r':
       default:
         return (num x, [int p = 0]) => x.toString();
     }
   }
 }