Chartjs-Why doesn't my Chart.js Canvas resize when I use flexbox?

Try to remove from .points_container-main and .bar-chart-container max-width, min-width styles. Add them styles in flex world:

.points_container-main{
  flex: 0 0 auto;
  width: 25%;
}
.bar-chart-container{
  flex: 0 0 auto;
  width: 75%;
}