web/web_widget_plotly_chart/views/web_widget_plotly_chart.xml

<odoo>
    <template id="assets_backend" inherit_id="web.assets_backend">
        <xpath expr="//script[last()]" position="after">

            <!-- Library -->
            <script
                type="text/javascript"
                src="/web_widget_plotly_chart/static/lib/plotly/plotly-latest.min.js"
            />

            <!-- Widget -->
            <script
                type="text/javascript"
                src="/web_widget_plotly_chart/static/src/js/widget_plotly.js"
            />

        </xpath>
    </template>
</odoo>