How to control the steps/labels on a Plotly Dash slider

Dash Python
1

I have created the following code and borrowed bits from the Dash website. The majority of the code functions as expected but there are no markers on the Slider and there are so many labels they overprint each other. I’d like the slider to go from -2000 to 0 in steps of 50 or 100, labelled as such. I assume I’ve done something wrong in the marks/step setup but I’m new to Dash and can’t figure out what it is.

import dash
import dash_core_components as dcc
import dash_html_components as html
from dash.dependencies import Input, Output
import plotly.graph_objects as go


Dep=363                 #Water depth (m)
Speed = 2.2             #Ship's speed (m/s)
ASV=1.5                 #Speed of sound in water (m/s)
SPI=6.25                #Distance between sample stations (m)
SB=0-((Dep*2)/ASV)      #Sound travel time to seabed (milliseconds) - negative to denote below time zero
IET=(SPI/Speed)         #Inter Event Time (s) time to travel SPI

Rmin=0
Rmax=-2000


external_stylesheets = ['https://codepen.io/chriddyp/pen/bWLwgP.css']

app = dash.Dash(__name__, external_stylesheets=external_stylesheets)

app.layout = html.Div([
    dcc.Graph(id='graph-with-slider'),
    dcc.Slider(
        id='3DHR Slider',
        min=Rmax,
        max=Rmin,
        value=(Rmax-Rmin)/2,
        marks={i: 'Label {}'.format(i) if i == 1 else str(i) for i in range(-20000,0)},
        step=200
    )
])


@app.callback(
    Output('graph-with-slider', 'figure'),
    Input('3DHR Slider', 'value'))
def update_figure(REC1):
    REC2= ((0-IET)*1000)-REC1  #UHRS record length - Function of above

    fig = go.Figure()
    fig.add_trace(go.Scatter(
        x=[0, 1,1, 0],
        y=[0, 0, REC2, REC2],
        fill='tonexty', # fill area between trace0 and trace1
        mode='lines', line_color='indigo',name="UHRS"))
    fig.add_trace(go.Scatter(
        x=[0, 1,1, 0],
        y=[0, 0, REC1, REC1],
        fill='tonexty', # fill area between trace0 and trace1
        mode='lines', line_color='blue',name="3DHR"))
    
    fig.add_trace(go.Scatter(x=[0,1],y=[SB,SB],name="Seabed"))

    return fig


if __name__ == '__main__':
    app.run_server(debug=True)