By creating Rickshaw.Graph({xScale: d3.time.scale()}), users can now
instantiate a Graph with a linear scale that has Date objects as
domain values. This way, a regular Axis.X can render time using D3's
time formatter:

    var xAxis = new Rickshaw.Graph.Axis.X({
        graph: graph,
        tickFormat: graph.x.tickFormat()
    });

xScale is coming from the configuration dictionary which may be
referenced by the Graph creator, or shared with other Graphs. We need to
ensure we copy the scale so that our mutations do not change the object
given to us.