There is two convention for showing current-potential graph. one is european and another is american. you can find more details in any electrochemistry book
Negative means the potential of your working electrode is negative with respect to your reference electrode (or just the counter if you have two electrode setup). Keep an open mind that oxidation processes can take place at negative potentials and reduction reaction can happen at positive potentials.
A final note: Oxidation/Reduction processes relates to the current. The direction of the electron flow through the electrode tells you weather the electrode is a cathode or an anode.(see links)
The words cathode and anode do NOT relate to polarization. Example, in a 3-electrode setup, during a potential sweep the current can be both positive and negative depending on which reaction that is taking place. As a result, the electrode is both an anode and a cathode during a sweep.
https://en.wikipedia.org/wiki/Cathode
https://en.wikipedia.org/wiki/Anode
In electrochemistry, generally one plots overpotential (eta) VS current density(i) - not potential VS current density.
In eta VS i plots, negative eta represents that reduction reaction is being studied. Positive eta represents that oxidation reaction is being studied.
I agree somehow with the answer of Pietro P. Lopes. But if negative is persistent all over the CV curve it is a reduction wave.
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