move title enc from binX options to mark options where other encodings are change d.x, d.y to d["day_of_week"], d["orders"] move your "days" number to text mapping into a separate cell and reuse the mapping in tooltip

the problem with marks misaligned: you are using rectY mark, which starts at the corner, you need to use barY mark, which is a specific case of rectY, but shifted to center now everything is broken because barY doesn't support the binX transform, you need groupX transform instead https://observablehq.com/plot/transforms/group

good use of tools: D3, O-plot and the chart cell charts work for their purpose and don't add unnecessary distortions this counts