main_plot = {
let svg = d3.create('svg').attr('width', width).attr('height', full_size)
let g = svg.append('g').attr('transform', `translate(${pad},${pad})`)
// Diagonal plots: first create group elements from the nested country time series data structure (or stacked time series)
// * Assign a `transform` attribute for each group element that will appropriately position the plot
// append a rectangle to each group that will function as each plot's border -> useful for debugging
// nested data join: this will correspond to each type of energy, for each country
// * Path elements should be created from this data join - 1 path element for each line/area mark
// * You should use the .values Array of data produced by nesting as the sequence of data items for the mark
// axes and labels for diagonal plot
// Correlation plots: first perform data join on Array of country pairs
// * Create group elements for positioning the plots
// Create a background rectangle for each group element -> useful for debugging
// Nested data join
// * Here you will derive a new Array within the `data` call, given the country pairing.
// * The returned Array should have all of the information necessary for creating heatmap rectangles.
// create legend for energy
return svg.node()
}