class canvasChart {
      constructor(canvas, data, colors=[], step) {
        this.canvas = canvas
        this.data = data
        this.colors = colors
        this.step = step
        this.ctx = this.canvas.getContext('2d')
      }
      init() {
        let maxH = Math.max.apply(null, this.data)
        this.canvas.width = this.data.length * this.step
        this.canvas.height = maxH
        this.data.forEach((element, index) => {
          this.ctx.fillStyle = this.colors[index]
          this.ctx.fillRect(index * this.step, maxH - element, this.step, element)
        })
      }
    }

let canvas = document.getElementById('canvas')
let cData = JSON.parse(canvas.dataset.rates)
let cStep = JSON.parse(canvas.dataset.stepWidth)
let cColors = JSON.parse(canvas.dataset.colors)
let cChart = new canvasChart(canvas, cData, cColors, cStep)
cChart.init()