duplicate horizon and rename it to line chart
a simple linechart implementation.
linechart now renders yaxis, quite dumb though.
be easy, no need to transform the actual linechart
add three configurable option to line chart

step
  when rendering how many data points will be rendered as one graph point, default: 1

stroke_width
  exposing svg.line's option, default: 1

show_yaxis
  render y axis, default: false
another useful option: summarize, a function that defines how data should be
summarized during a certain step. The default summarize function is sampling
the first element in the step. d3js also provices a series of other candidate
like d3.sum, d3.max, d3.min, d3.median, d3.mean