Histogram data requires API v973.02+ and TWS 964+. Instead of returned data points as a function of time as with the function LYNXApi.EClient.reqHistoricalData , histograms return data as a function of price level with function LYNXApi.EClient.reqHistogramData and data is returned to LYNXApi.EWrapper.histogramData

** Python **

def histogramData(self, reqId:int, items:HistogramDataList):
    print("HistogramData. ReqId:", reqId, "HistogramDataList:", "[%s]"
          % "; ".join(map(str, items)))

** Java **

    public void histogramData(int reqId, List<HistogramEntry> items) {
        System.out.println(EWrapperMsgGenerator.histogramData(reqId, items));

An active histogram request which has not returned data can be cancelled with LYNXApi.EClient.cancelHistogramData