def parse_tstv_by_qual(self):
        """ Create the HTML for the TsTv by quality linegraph plot. """

        self.vcftools_tstv_by_qual = dict()
        for f in self.find_log_files('vcftools/tstv_by_qual', filehandles=True):
            d = {}
            for line in f['f'].readlines()[1:]: # don't add the header line (first row)
                key = float(line.split()[0]) # taking the first column (QUAL_THRESHOLD) as key
                val = float(line.split()[6]) # taking Ts/Tv_GT_QUAL_THRESHOLD as value
                if (val == float('inf')) or (val == float('-inf')):
                    val = float('nan')
                d[key] = val
            self.vcftools_tstv_by_qual[f['s_name']] = d

        # Filter out ignored sample names
        self.vcftools_tstv_by_qual = self.ignore_samples(self.vcftools_tstv_by_qual)

        if len(self.vcftools_tstv_by_qual) == 0:
            return 0

        pconfig = {
            'id': 'vcftools_tstv_by_qual',
            'title': 'VCFTools: TsTv by Qual',
            'ylab': 'TsTv Ratio',
            'xlab': 'SNP Quality Threshold',
            'xmin': 0,
            'ymin': 0,
            'smooth_points': 400, # this limits huge filesizes and prevents browser crashing
            'smooth_points_sumcounts': False
        }

        helptext = '''
        `Transition` is a purine-to-purine or pyrimidine-to-pyrimidine point mutations.
        `Transversion` is a purine-to-pyrimidine or pyrimidine-to-purine point mutation.
        `Quality` here is the Phred-scaled quality score as given in the QUAL column of VCF.
        Note: only bi-allelic SNPs are used (multi-allelic sites and INDELs are skipped.)
        Refer to Vcftools's manual (https://vcftools.github.io/man_latest.html) on `--TsTv-by-qual`
        '''

        self.add_section(
            name = 'TsTv by Qual',
            anchor = 'vcftools-tstv-by-qual',
            description = "Plot of `TSTV-BY-QUAL` - the transition to transversion ratio as a function of SNP quality from the output of vcftools TsTv-by-qual.",
            helptext = helptext,
            plot = linegraph.plot(self.vcftools_tstv_by_qual,pconfig)
        )

        return len(self.vcftools_tstv_by_qual)
Create the HTML for the TsTv by quality linegraph plot.