The AI Index tracks a number of relevant economic indicators, and their very recent 2018 report provides graph data for this question on page 32 of their 2018 report. The amount of VC funding for AI startups in the US, measured in USD, keeps growing, and at a rate twice as high as the growth rate of VC funding across all sectors.
The exact methodology for estimating the number, relying on Crunchbase and the Dow Jones VentureSource database, can be found in Appendix 10 of the 2018 report.
Data
The AI Index also has a public data Drive but unfortunately it doesn't contain the exact numbers for this question, so you'll have to sharpen those eyeballing skills!