The "USDx Chart MT4" indicator plots an offline chart, on which the USD index is drawn instead of the usual bars and candles. That is, the "USDx Chart MT4" indicator plots the chart of the artificially created USDx symbol.

The formula for the USD index calculation uses the data on "EURUSD", "GBPUSD", "USDCHF", "USDJPY", "AUDUSD", "USDCAD" and "NZDUSD" symbols. The formula for calculating the USD index:

(100*MathPow(OHLS_EURUSD.ohls_open[i],0.125)+100*MathPow(OHLS_GBPUSD.ohls_open[i],0.125)+
                 100*MathPow(OHLS_USDCHF.ohls_open[i],0.125)+100*MathPow(OHLS_USDJPY.ohls_open[i],0.125)+
                 100*MathPow(OHLS_AUDUSD.ohls_open[i],0.125)+100*MathPow(OHLS_USDCAD.ohls_open[i],0.125)+
                 100*MathPow(OHLS_NZDUSD.ohls_open[i],0.125))/8.0;

The use of the USD index implies the consideration of a D1 chart, therefore the USD index chart is plotted only once - based on the history and is not updated online.

More details on the plotting of the USD index chart can be found in the article Writing a Market indicator for any non-standard chart.