Trouble adding second X axis variable correctly!

Ah hello.

I am having problems with creating a chart from some simple data.  I have a series of TCP SEQ numbers (basically the ID of a packet sent on the internet) and a series of TCP ACK numbers (basically an acknowledgement of data received).  Both these sets of values have a time at which they occurred.  

In short, what I want is to plot both the ACK and SEQ numbers on the same graph, where the X axis is the time, and the Y value is the SEQ/ACK value.  if you open the attachment, you can see I have the SEQ values and their timestamp in black, and the ACK values and their timestamp in red.  The graph looks good, but when you click the lines, you can see the ACK line is good, but the SEQ line is using the SEQ values (which is fine) but the timestamps for the ACK data!  I want the SEQ line to use the SEQ timestamps, and the ACK line to use the ACK timestamps!

Can someone please tell me what I'm doing wrong?

MyAttempt.xlsx

TIA!