`columns`:

1. `datarows` should be of type Array of Arrays, representing list of rows.

2. `columns` should also be of type Array of strings, representing column headers.

@group Urth Viz
@element urth-viz-pie
-->

<dom-module id="urth-viz-pie">
    <link rel="import" type="css" href="../nvd3/build/nv.d3.css">
    <link rel="import" type="css" href="../urth-viz-chart/urth-viz-chart.css">

    <template>
        <link rel="stylesheet" type="text/css" href="../nvd3/build/nv.d3.css">
        <link rel="styles