Within each slide you can provide a chart title value and this will be rendered in the chart and in the downloaded presentation.
Example in JSON
{"chart_title": "custom title"}
Example slide JSON with Chart Title
{"base_settings": {"file_name":"test_demo","company_template":"PPTX.Template.PsH.TEST.-.sig2.pptx" },"body": [{"chart_title":"custom title","table_text":"Demo","question_text":"What is Demo?","base_text":"A Demo is something that is used for presentation!","rows": ["Cardiology","Endocrinology / Diabetology","General Practice / Family Medicine","Internal Medicine","Other"],"columns": [{"text":"Total","items": [{"text":"Total n=100","base":100,"data": [16,5,37,42,0] }] }, {"text":"Gender","items": [{"text":"Male","base":100,"data": [16,5,37,42,0] }, {"text":"Female","base":100,"data": [20,15,47,22,10] }] }],"view_settings": {"slide_master_index":0,"slide_layout_index":0,"placeholder_index":0,"chart_type":"bar","show_chart_title":true,"chart_title_font_bold":false,"chart_title_font_size":12,"chart_title_font_color":"rgb(123,123,123)","chart_title_font_family":"arial_narrow" } }]}