The graph shows first 4 weeks only. That means the early stage of infection in China when the virus basically spread unhindered; if you extend the curve to afte

The graph shows first 4 weeks only. That means the early stage of infection in China when the virus basically spread unhindered; if you extend the curve to after emergency was declared nationally then you would absolutely see the effect of masks. Edit: First 4 weeks after the 100th case actually. But the numbers are clearly delayed and reflect transmission that happened much earlier, and you can already see the curve leveling off in the graph.