I have had a look online about bridge taps and none seem to look like mine, as others seem to have one v dip where as mine seems to continue from the start of the graph to the end. So can anyone help explain why my graph looks like this?
Short answer: your dip appears to be caused by a long length of extra wire (I calculated 66m); other examples may well be shorter lengths.
Long answer:
As others mention, the technical explanation is that the dip is caused by reflections from the far end of the extra bit of wire; the frequency the first dip is seen at depends on the length of that wire, and the wavelength of the matching frequency; Table 1 in that linked PDF gives you an idea of the relationship.
Whatever the first frequency (or tone number N) the dip first appears on, there will be repeats at 2N, 3N etc. This is a kind of resonance effect - whatever length of wire is an exact match (in terms of whole wavelengths) for frequency F is always *also* going to match 2 wavelengths of half that size (ie 2F frequency), and 3 wavelengths of a third that size (ie 3F frequency).
In figure 2 of the PDF, the example shows a dip at tone 1400. By table 1, that means the tap is 8m long. It also means there will be further dips at tone 2800, 4200 etc ... but the display just doesn't show them.
Your picture has the first dip at about tone 164, which is too low for table 1 to help - suggesting the tap is longer than 46 metres, up to maybe 100m-ish. However, table 2 does apply, even though the text describing how to use it (at the end of page 6, "Occasionally, this is too difficult ...") can be hard to follow.
The way to use table 2 is to see that your first dip is at tone 164, and your third dip is at tone 834. The difference between tone 164 and tone 834 (or tone delta) is 670. Using the column titled "1 sub-dip" (because there is one sub-dip between the first & third), we can see the tap is of a length between 66.3m and 71.3m