If those "blank" cells aren't really blank (contain space or other non-printing characters), Excel will treat them as text and parse the data differently than you would expect.
Select the plotted series and look at the series formula to see how Excel parsed it. I'll bet it looks something like this:
=SERIES(Sheet1!$D$1:$D$220,Sheet1!$A$221:$C$500,Sheet1!$D$221:$D$500,1)
Where the first three columns are used for X values, which doesn't work in an XY chart (though formatted as date-time, the first X value in your chart is zero), and the top at least 218 cells in column D are used as the series name (the chart title is "SELL 433" which are the values in D1 and D218 the first number found in column D, and the red outline that highlights series names surrounds all of the visible part of column D in the first screenshot).
You would expect instead three series like this:
=SERIES(Sheet1!$B$1,Sheet1!$A$2:$A$500,Sheet1!$B$2:$B$500,1)
=SERIES(Sheet1!$C$1,Sheet1!$A$2:$A$500,Sheet1!$C$2:$C$500,2)
=SERIES(Sheet1!$D$1,Sheet1!$A$2:$A$500,Sheet1!$D$2:$D$500,3)
To fix this, press Ctrl+G (GoTo), Special Cells, Constants, Text, then press Delete. Now rebuild the chart.
![Go To Special](https://cdn.statically.io/img/i.sstatic.net/Iy66a.png)
=A2+0
in G2. Then copy and paste G2 across G2 downto K1000.