width of curve?

Hello, I have two sets of data (listed below). Y is logarithmic scale base10 (0.01%-100%). I am required to plot a line on the graph corresponding to 5% of Y & then measure the width of the curve at that line. I am completely lost I've never used charts in excel, any help would be wonderful!

X
-0.342020143
-0.309016994
-0.275637356
-0.241921896
-0.207911691
-0.173648178
-0.139173101
-0.104528463
-0.069756474
-0.034899497
0
0.034899497
0.069756474
0.104528463
0.139173101
0.173648178
0.207911691
0.241921896
0.275637356
0.309016994
0.342020143

and

Y
0.12%
0.14%
15.20%
29.53%
45.34%
59.12%
71.28%
82.16%
88.68%
95.61%
100.00%
95.95%
89.70%
79.56%
66.72%
54.56%
39.49%
22.80%
6.22%
0.05%
0.03%