Question

Needing some guidance about how to properly graph data that is very small and stored as BigDecimal.

If anyone has had experience using BigDecimals in any graphing scenario I believe your input on how you placed these into a usable sequence would be invaluable.

Presently I’m using lazy_high_charts and it really seems that this is going to work out exceptionally well, however I’ve run into a hitch where I’ve not dealt with data on the minute BigDecimal scale hitherto.

Given my queries, I’ll be pulling out about a 1,000 data points for a few different series of data ranging in accuracy from about 0.100E-9 to about 0.100E-1.

What would be the best way to prep these data series for presentation in such a graph that has a scientific application and therefore precision is important? I’m not sure if I could or should continue in BigDecimal or something else?

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I’m presently querying the database with a line similar to:

series_a = dataset.order("date").select('data_set.data1').limit(1000).all.zip

I’d appreciate some guidance of going from this result (again, the output is an array of BigDecimals) to the appropriate format for what will go into the chart series.

An contextual example of the code I’m using to build the chart in my controller is:

@h = LazyHighCharts::HighChart.new('graph') do |f|
  series_a = dataset.order("date").select('data_set.data1').limit(1000).all.zip
  series_b = dataset.order("date").select('data_set.data3').limit(1000).all.zip

  f.series(:name => 'Data 1', :data => series_a)
  f.series(:name => 'Data 2', :data => series_b)

  f.chart({:defaultSeriesType => "line" })
  f.yAxis [
        {:title => { :text => "Left Y Label", :margin => 10} },
        {:title => { :text => "Right Y Label"}, :opposite => true }
  ]
  f.xAxis(:title => { :text => "X Label"} )
  f.title(:text => "Graph Title")
  f.legend(:align => 'right', :verticalAlign => 'top', :y => 75, :x => -50, :layout => 'vertical')
end

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I think I’m a bit further along in my understanding of this having decided to convert the BigDecimals to strings as per “How to access fields by attribute name from ActiveRecord row results?” However it appears ultimately this fails and gives erratic results as the series functions :data field expects, I think, numeric input. I am thinking that this ultimately emits json to highcharts, however I’m still stuck for the moment in my attempt to pass these values on in correctly.

Answer 1

When you do this:

series_a = dataset.order("date").select('data_set.data1').limit(1000).all.zip

you’ll end up with an array of arrays as we already discussed elsewhere. If you get rid of the zip, you should end up with an array of objects in series_a. I’d guess that chart would be happier with an array of numbers so:

series_a = dataset.order("date").
                   select('data_set.data1').
                   limit(1000).
                   all.
                   map(&:data1)

or, if data1 is a BigDecimal (due to using a fixed precision type in the database) then maybe you’d want this:

series_a = dataset.order("date").
                   select('data_set.data1').
                   limit(1000).
                   all.
                   map { |o| o.data1.to_f }

to get an array of floating point values that the chart should know what to do with.

I’m not familiar with Highcharts so there is some guesswork here.