so you are suggesting connecting the dots between the sales
Yes. Imagine the current price plot above if you deleted the zero-sale points. The line would then connect the closest days with sale.
Rather than draw an empty dot (a circle with a white interior), I think you can just omit the point entirely. Whether there is a point would be a bit more clear if the point size were increased.
That might be better, cause otherwise we will see this spikes when a trade occurs, rather than a line chart.
Yeah, if you fill prices with the last-sale price, you'd get more of a plot that looked like it had steps. I sometime use "step" plots, but it really depends on what we think is the most accurate reflection of the data.