Create a grouped bar chart comparing advertising efficiency by channel.

Use dual Y-axes:
- Left: ROI (%)
- Right: ROAS (x)

Show two bars per channel: ROI and ROAS.

Use a clean, professional analytics style:
- Muted blue (ROI) and muted teal (ROAS)
- Subtle vertical gradients
- Light neutral background
- Minimal gridlines, no decorative effects

The goal is to identify the channel with the highest return on ad spend.